Tre: derived from the french word tres meaning (and substituted for) the word 'very'. Except, in this context it is written phonetically and you leave out the s. (The irony makes it hip)
Book: derived from t9 predictive texting. When attempting to type the word 'cool' but t9 recognizes it as 'book'. People got lazy (people in Davis, Ca, to be exact) and left it as is, substituting 'book' for 'cool' in texts. trend eventually spread to spoken word.
-Often (but not exclusively) used sarcastically when apathetically humoring another person, but really the person doesn't give a shit.
*not to be confused with, and not in association with Green Day's drummer Tre Cool*
-"Thanks, I got it from American Apparel, half off!"
"Clear your schedules, cause i'm coming to town and it's gonna be tre book!"
"cool mumu. is that from like Urban or something?"
-"uh, no.. its from Guatemala
"international, it's definitely tre book."
"hey man wanna go smoke some potsmarijuana!??"
-"yeah totally!!...tre book"
WHAT IS TRE BOOK??? I KEEP SEEING IT ALL OVER FACBOK