What, nobody has mentioned The Straight Dope? What more would the heathens need? Maybe a 2002 encyclopedia. How about the Hitchikers Guide to the Galaxy?
I guess I’d solve all of our problems by sending a book on reptile and pest removal to Adam and Eve.
D’Oh! MEBuckner beat me too it, sorry, I didn’t click the link.
::::humbly bows before the mod::::::
D’oh!
I forgot, Benjamin Franklin had some other thing I want him to have done! Can you imagine him, down in his basement sending “Franklin Code” messages to his geek buddies in Alexandria?
Revolution? Naah, I got to troubleshoot this breadboard hook up that Jefferson sent me.
I better pick someone innocuous.
Tris
It worked pretty well for the publishers of the Hitchhiker’s Guide To The Galaxy:
“The building is home of a book, and was built here on the proceeds of an extraordinary copyright lawsuit fought between the book’s editors and a breakfast cereal company.”
and:
"The editors, having to meet a publishing deadline, copied the information off the back of a packet of breakfast cereal, hastily embroidering it with a few footnoted in order to avoid prosecution under the incomprehensibly tortuous Galactic Copyright laws.
It is interesting to note that a later and wilier editor sent the
book backwards in time through a temporal warp, and then
successfully sued the breakfast cereal company for infringement
of the same laws."
I’d send a copy of the current year’s Kovel’s Guide to Antique and Collectibles back to 1967 to my dad, so I could grow up rich.
jayjay (hey, I’m no Schweitzer, okay?)
You want to send a textbook that a) conveys useful information (probably medical) and b) provides a framework for scientific investigation so that the recipients can learn to innovate on their own.
I might acquire a nursing manual in Dutch and send it to Amsterdam c. 1375. By that time, the inhabitants would be familiar with canal locks and windmills and would therefore have an appropriately inquisitive outlook.
There may be modern texts translated into Ancient Greek. If so, I trust that Aristotle could make good use of them. Note though that tinkering was less prevalent in ancient times than during the medieval period: to lean against this tendency, I would avoid sending advice on civil engineering.
If the modern text was written in Latin, I wonder who the appropriate recipient might be.
You said “one book”, not “one volume”. So send 'em an encyclopedia. 
How about sending The Handbook of Chemistry and Physics back to Isaac Newton? It wouldn’t be of much use before that without Calculus (well, the reference tables would be valuable).
Let’s think about events we want to avoid, and which book would be best to do that. The dark ages, the Inquisition… You can’t just send a science book back in the middle of that, because it’ll just be considered a heretical document. Youv’e got to get to the people before they take that particularly disastrous turn in history.
How about we send a laptop with the ability to access the Internet in our time?
On second thought, they’d probably just look at porn.
How about sending a copy of “Left Behind” to Jesus?
jayjay
I think I’d send a school exercise book of my tortured, confused, mistake-ridden Latin translations of Pro Milone back to Cicero. Might make him choose slightly clearer words the next time. Or not bother speaking at all.
The Joy of Sex would be a nice gift for Queen Victoria.
Ah yeah… maximum good for mankind… umm, then “The Pyromaniac’s Guide to Firestarting” to OG in Cave 5, pre-Neanderthal times. Get civilisation kick-started a bit earlier.
I’m surprised that no one has suggested sending a detailed world atlas back in time. It potentially circumvents a lot of the language problems, so long as the recipients are familiar with the concept of a map. You’d just have to bookmark the “you are here” page, so that they would “get” the idea without having to be able to read the text.
Imagine, if you will, what Alexander could have done with maps of the world at varying scales.
-Ben
A biography on Hitler to the chancellor of Germany before 1933, thereby making WWII non-existant.
Good God, a rare double-whoosh.
How about doing the opposite? What (lost) book would you lie to send FORWARD to the present time? The plays of Sophocles, for example.
Hitler did not try to hide his ideas. They voted him all the same.
I would send the ancient romes an instruction manual to allow them to anticipate guttenberg by hundreds of years. At the very least we could read the whole History by Tito Livio.
Just about everything in the Great Library of Alexandria, I guess.
The Second Volume of the Poetics of Aristotle, of course. Watch out for murderous Benedictines, though.
Tris
Send back a good military first aid & camp sanitation manual.
Or, send a book on building sailboats–to the Aztecs. Or the Maya. 500 years before Colombus. Let’s see who invades who, white-eyes! 
I would send a XVIII° century * technical encyclopedia (complete with schematics) translated to a relevant written language (probably Chinese) to Gengis Khan. So, we would perhaps live now in an unified and peaceful world.
(*I assume that a more recent encyclopedia describing modern technology wouldn’t be understandable and of no use at GK times)
PENITENZE AGITE!!!