It must not have been published when you were 21. But, if you choose the first book in a series, your 21 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 The Warrior’s Apprentice, by Lois McMaster Bujold.
I’m a big fan of the 2007 book Mistakes Were Made (But Not by Me) by Carol Tavris and Elliot Aronson, and would guess that I could have profited from knowing its contents when I was 21.
The Conquest of Happiness by Bertrand Russell. I just think that, and branching out from that to some of Russell’s other writings, might have made my early 20s easier.
Lord Foul’s Bane by Stephen R. Donaldson. Since I turned 21 in 1975, and the original Covenant trilogy was published in 1977, I’d have only needed to wait 2 years for the rest of the trilogy.
Regardless of what one thinks of the Covenant books and Stephen R. Donaldson generally, the Covenant trilogy was the story I really needed to help find my way out of the peculiar lostness I was in during the mid to late 1970s. I didn’t actually come across it until 1981, and it was still the story I needed then. Wouldn’t have minded some help in getting my head straightened out a few years earlier.
Can we make it my 10th year old self? I think she would have benefitted from Cordelia’s Honor a lot more than the 21yo version. I spent so much of 5th grade hearing what an enormous problem with authority I had… I don’t have a problem with authority, but with lack thereof, once you take into account that one of that word’s meanings is “knowledge, expertise”. So, yeah, I don’t want power. I just object to idiots having power over me.