So… I have two books by an amazing African American author named Walter Mosely and they are both first editions. The first one was printed in 98’ and the second in 99’. That’s 1998 and 1999 for those that may need a little more clarification. My question is, when do first edition books become valuable or have any value at all other than the price paid for them? 
When you can get someone to pay you for them.
Otto has it right. Books become collectible when someone wants to collect them.
Don’t forget that most books never go into a second edition at all. Which is the basis for this classic riposte:
Even books that do have second editions often have first editions so large that they aren’t very valuable. Moseley was already a well-known author by 1998. His first books may have had small first editions, and those are the ones that become valuable.
Other factors: modern first editions have to be in practically perfect shape to be worth anything. The smallest nick, scratch, or stain will greatly reduce the value. You must have the dust jacket. It can’t be a book club or ex-library copy. And signed books are worth a great deal more than unsigned copies.
And they have to be the popular books. Since the two books from 98 and 99 are from the Socrates Fortlow series, rather than the Easy Rawlins series, they probably will never have as much value.
Sorry.
I have signed first-edition copies of all 7 Harry Potter books. How much do you think they’d fetch?
Considering how popular Harry Potter is… well, your guess is as good as mine.
(Btw… my guess is some where in the tens of thousands or even higher.) 
So, how does the series turn out? Is the V-Man really Harry’s father?
First editions from what country? The British will be worth more than the American.
I seem to recall seeing a first edition of one of the HP books on an episode of Cash in the Attic (BBC auction show) from a couple years ago that was estimated at 60 pounds. It didn’t sell at auction.
If you want to see what the asking price for any books you have, just log onto somewhere like Abebooks . There you can see what book-sellers all over the world are charging for any rare , first edition or signed copies. The web site contains a very good search facility which enables you to narrow down the list to include only such things as first editions.
chaoticdonkey writes:
> I have signed first-edition copies of all 7 Harry Potter books. How much do you
> think they’d fetch?
Not nearly as much as the cost of the time machine you used to get the 7th book.
Yes. The series ends thus:
“Harry,” Voldemort wheezed, as though he were using some sort of respirator, “I am your father.”
Don’t the Harry Potter books have really really huge first edition printings? Like 30 or 40 million books? I would think that only the first book would be valuable at all.
You might have a valuable Harry Potter and not know it. In the first UK printing of “Goblet of Fire” there are three errors , later corrected. To quote the Abebooks site
*errors on page 503 (“Dumbledore, come!” said Crouch angrily - was later corrected to Fudge), page 579 (James Potter exits Voldemort’s wand before Lily - the order was later reversed) and page 594 (“The Imperius curse” Moody said - was later corrected to Crouch said)
*
The going price of these books is between £100 and £250.
To answer the original question about the Walter Mosely books. Again according to the Abebook site, $20 seems to be the going rate for first editions of these books on sale in the USA.
You’ve gotten quite a few good answers, ayla106, and Otto’s is the most clear and concise. To expound a bit…
The value of a collectible item of any kind is based on demand and scarcity. Some first editions are never in demand (I’ll happily ship you a first edition of my first book for half the cover price). Other first editions are never scarce (such as the Harry Potter books, with first printings of over 10,000,000 copies).
To clarify a bit of terminology, the real value lies with the first printing, not the first edition. One of my recent books is still in its first edition (i.e., no revisions have been made), but it’s in its third printing (i.e., the publisher has printed three batches of them).
Identifying collectible first printings/first editions can often be difficult. Book club editions, which are basically worthless, are often darned near indistinguishable from true firsts. They may be printed with the same printing plates, and only a small mark or design on the cover or title page sets them apart–sometimes not even that.
To dispel another myth, age does not imply value. I have some books in very good condition from the mid-1800s that are worth a couple of bucks each. I have a signed first from 1984 that’s worth about $300.00. It’s all about scarcity and demand.
A final note about limited editions and collector’s editions: don’t count on them to be valuable. I have one limited, numbered, signed first edition (one of 1,500 printed) that’s worth some money. On the other hand, I have a collector’s edition of the fifth Harry Potter book that you can buy on eBay for about half what it cost new. It was a “limited edition,” all right. Limited to 100,000 copies! That may be less than 1% of the first printing, but it’s still not few enough to be considered scarce.
Why are book club edition books considered worthless? That’s something I’ve always wondered.
Partly because they aren’t considered first editions, which only come from the publisher.
From a pure value standpoint, book club editions are usually more cheaply made.
What RealityChuck said, plus the book clubs tend to print a pretty big press run.
They frequently use cheap paper and print in a smaller size, too, which actually makes them easier to spot.
I don’t profess to know a great deal about book collecting in any genre except science fiction, but there are some BCEs that are quite valuable - particularly if the first edition was a paperback. You see, there are people (like me) who prefer a hardcover. Now, I might own a PB first, but for reading, I prefer a HC.
I’ll give you an example of a fairly valuable BCE: Roger Zelazny’s Jack of Shadows. I just did an ABE search. Counting both BCEs and trade editions, there are only 20 listed, the cheapest at $18, with one BCE listed at $50. The cheapest trade edition is an ex-lib, for $20. The cheapest non-ex-lib trade edition is a UK edition (not the “true first”) for $60. The cheapest true first that sounds like something I’d want (if I had the money) is listed at $85. Lloyd Currey (the expert on SF collecting; he literally wrote the book) is asking $150 for what sounds like a fairly desirable copy. However, he also has two other copies listed at $300 and $325. Currey’s prices are always fair; unlike many other used booksellers, he doesn’t go looking for “all the market will bear” - the most expensive copy is listed at $350, and by the description, it’s not in as good condition as the one Currey has listed for $300. This is only one example. Early in his career, Zelazny’s books didn’t get very big HC printings, not even the BCEs. There are other examples, but you’ll forgive me if I don’t just spill all I know. Other Zelazny books - especially his Amber series - are very easy to find.
One other thing that should be pointed out: the spread of online selling - both via eBay and other big sites (e.g., Amazon, Powell’s), as well as ABE and other hosts/aggregators which list books for the bookstores or private owners who wish to sell them - has resulted in the sales price of most books going down. You see, many extant copies are no longer “lost”; they can be listed online to reach a wider audience. At first, this resulted in some price increases. I remember seeing that in the late '90s. But then as internet use began to spread, people began to realize that there were probably more copies out there somewhere, no matter what book it was. In the end, unless the book is old, and/or had a short initial print run.
As to the discussion above about the Harry Potter books - the most valuable ones are the UK printings. Even though they are now printing the books simultaneously in all places, the UK is still considered the “true first”. Under normal conditions, the true first is the first printing, wherever it’s done, whatever the type (paperback or hardcover). However, in simul-printing, the author’s home country’s printing usually counts. In Rowling’s case, since she writes the books using English vernacular and slang, it is “translated” for printing in North America (although it Canada there are two firsts - one in Brit-speak, one in Ameri-speak).
Why did I choose to use the Zelazny book to illustrate? Because I lucked onto a BCE of this book online for $5.50 (including shipping). Did I buy it? You bet!
There is a word for someone who buys a $50 Zelazny Book Club edition: sucker.
Going to bookfinder.com (a metasearch site that combs far many more places than just abebooks, and brings up over three dozen listings) shows the difference in value between the true first edition and the ex-library and book club editions.
I suppose it’s nice that people are getting such money for BCEs (or are they?), but this is just an artifact of the Internet. If you go back in several months, check how many of these books are still being offered. These prices won’t last; the suckers will lose money, and true firsts will grow ever more valuable