Bibles sell well. Dianetics dont until the Scientologists come buy and buy the whole lot.
Another anecdote or two: We downsized when we retired and absolutely nobody wanted books, donated or otherwise, except for the Salvation Army. That includes second hand bookstores and the local library.
My brother in law just died. He had a pretty comprehensive collection of books on Montana history. The library in his hometown of Sheridan took them as a donation and paid for the shipping from Alaska. But these were books addressing a specific niche.
I’ve sold books on Alaska on eBay, but again, that’s a niche market. A general library with common books is pretty much an albatross around your neck.
If they’re sold to the $cientologists, that’s still sold. I’m referring to their being bought but not read.
Bibles are a frequent gift throughout the life of many Christians, but chances are they stick w/ the copy they prefer to read if they choose to read it at all. I had 5 just in English at one point and never read any other than my Ryrie study bible and I’m only 46. My grandmother had over 30 when she died at 81 and that was **after **each of her 7 surviving kids picked one for themselves. Those still shrink-wrapped were given to her church.
There’s probably a bunch of people who’ve gotten Dianetics as a gift in hopes they’ll join up; no telling how many of those were read, but I’ll wager not many.
Totally? Every single word? I never thought I’d ever achieve that pinnacle.
This is a link to a story from the public radio program On The Media about a used-book wholesaler in Maryland (Wonder Books, for those in the area) and how they sell some books that no one is buying to read. So if, for example, you want that elegant library look, they’ll sell you leather-bound books at a rate of $350 a linear foot. In other words, if you want to decorate a ten-foot wide bookshelf, it’s going to cost you $3,500. If you want all beige books, to match the decor, they’ll sell you that. If you just want to fill a bookcase with books that make you sound smart, they’ll select and send you a selection.
Here’s another story about books as decor. These people will even make custom dustcovers for all of the books so that the whole collection looks like a mural.
I’m sure that most SDMB readers are aghast at the idea of people buying books not to read, but there is an audience for this sort of thing. (And I’ll bet that some people living in Great Houses like the one in Downton Abbey were putting together those awesome libraries of first editions more because that’s the sort of thing to do than because they actually wanted to read the books.)
I wish I’d known about this place when I was in the area about six weeks ago as I love books. However, I do think they’re somehow the exception. I’ve no idea how they’re doing it because every used bookstore I know of around here has closed, with the exception of a local chain called Half-Price Books. And they’ve closed several locations.
I sold books on eBay for a long time, but quit doing so several years ago. It’s not worth it. Books have almost no monetary value anymore. I’ve been selling my mom’s gigantic collection of sci fi which do well in lots, but that’s it.
To the OP, I agree with the majority here that it’s most likely not worth your time to research your relative’s books. If there are any specialty books you might consider contacting someone within that specialty with the intent of giving them the books. For example, my second cousin was very much into birding, so her family gave her books to the local version of the Audubon Society. World War II? Veterans will love them. Knitting or crocheting? Find a local club. That kind of thing.
Good luck.
Day ain’t over yet.
If you’re lucky. Don’t just dump them at the library, ask the person handling book sales if they think they’ll sell. Most books won’t.
I’m a used bookseller who also helps manage my library’s Amazon account.
Check the books for money and signatures (and things like credit card receipts, photographs, mammogram results :o , and other personal items) and if you suspect something may be valuable, especially if it’s a niche interest, look it up on Amazon. If you have a used bookstore in your area whose stock isn’t composed of Stephen King paperback and romance novels, take them there and have the owners look at them. An auction house may also be able to help you, especially if you do have something rare and/or valuable.
With a few exceptions, best-sellers from past decades aren’t valuable. However, it never fails to boggle our minds at what people want (or don’t want). The people at the library or resale store can make that decision.
Dewey Finn, some community theaters want books to use as a backdrop. It doesn’t hurt to ask.
We’ve found that college-level engineering books are worth listing on Amazon no matter how old they are (the information never becomes obsolete) and recently got some calligraphy books by a local author, now deceased, that turned out to be quite valuable. We later found out that the author, who was a professor at a local college for several decades, was considered THE worldwide authority on calligraphy, and he was the kind who taught teachers. He also knew that 99.9% of the people who took his class were doing so only for the art credit, but he didn’t mind. Some old cookbooks and children’s books are worth something to collectors. You just never know.
p.s. With a few exceptions, encyclopedias and coffee-table books from Time-Life or the Reader’s Digest are worthless. Sometimes schools want them for art classes; same with RD Condensed Books.
p.p.s. Unless you have a second copy of the Voynich Manuscript or a page from the Gutenberg Bible, that kind of thing, anything with black mold MUST be discarded. 
We have the Book Rack around here, and they basically specialize in last year’s bestsellers. They also have a lot of children’s books, including homeschooling materials, and sell books by local authors, most of which were self-published. There are a few hole-in-the-wall used book stores and one larger store, which has been there for almost 100 years, and they stay afloat by selling music and DVD, and also has an Amazon account.
No, because what it means is that there are so many books on planet Earth that everyone who wants some already has as many as they want.
I once took a lot of military books and gave a tax receipt. The individual was required to provide two independent assessments of the value by military book experts.
No money, but the tax receipt for a donation to the Crown.
If they are on a certain subject, a special-interest group or museum may be interested.