(Heh, I often talk like I, myself, am a sim, e.g. ignoring homework because my "fun" bar is too low. Hehehe)
WHAHAHAHAHA!!!! If you are a sim I've already discovered one trait of you now: good sense of humor.
But yes, masterpieces give an extra moodlet, besides the fact that Sims, especially bookworm Sims, can have more fun off it because they took longer to finish. It isn't really my first choice when I need my sim to read 50 books in order to write faster, but for carrying it with you, on a long trip from home, it can provide you some easy (not too expensive) fun.
[Edited for correct quoting. - Swede]