It must not have been published when you were ten. But, if you choose the first book in a series, your ten year old self won’t be able to read the rest of the series until it’s really published. And your past self won’t be able to tell anyone else about the book.
I would choose Castle Hangnail, by Ursula Vernon.
The Making of Microsoft: How Bill Gates and His Team Created the World’s Most Successful Software Company would certainly be useful in the late 1970s, precocious investor that I was, though I did miss out on making a bundle on Ideal Toys when they got the Rubik’s Cube.
One of the Julia Donaldson books, simply because they’re fun. Either The Gruffalo or Tiddler.
One of the Captain Underpants series, or Babette Cole’s Dr. Dog.
I’m surprised you didn’t forbid the sending of reference books. Thanks to the plot from Back To The Future II, I’m sure I could find a suitable book of statistics.
If I can weasel with the terms a bit, I’d send my 15-year-old self Stephen King’s On Writing: A Memoir of the Craft.
I’d send back that book from Phys Ed class, seventh grade, the sex ed book. That way I wouldn’t embarrass myself when I read about intercourse.
Who Moved My Cheese?
An omnibus volume of the Harry Potter septology, along with a note to type it up and submit it in manuscript form (in seven volumes, of course) to a publishing house starting in 1986.
‘Even Cowgirls Get The Blues’, by Tom Robbins
It would have been a brilliant read for that period in my life I think.
You must have been a pretty precocious ten-year old…
Catch-22. Published in 1961 when I was eleven. First read it in my late teens. I’m not sure I would have appreciated it at ten, but it wouldn’t have hurt.
“Gray’s Sports Almanac 1950 - 2000”
Macdonald Hall Goes Hollywood by Gordon Korman, because I loooved the Bruno and Boots books when I was ten and would have been thrilled to get my hands on a not-yet-published one; and because, alas, they turn out not to be anywhere near as funny if you read them as a grown-up, so that one was sort of wasted on me when it did come out. (I might be able to come up with a more serious and life-changing answer if the OP had picked a different age.)
Notes From Underground - Dostoevesky
The Outsider - Colin Wilson
Darkness At Noon - Arthur Koestler
Gulag Archipelago - Alexander Solzhenitsyn
Hunger - Knut Hamsen
Wow - Darkness at Noon hadn’t been published when you were 10 years old? You are way older than I had guessed.
Gulag Archipelago at ten?
Colour me impressed!
Indeed, I have completely fucked up on the whole chronology thing.
Ulysses. I’m a mean bastard
Wee Free Men. Even though I wouldn’t get to read the rest of the series for years and years, the first tale of Tiffany Aching would be worth it to me.