1) Maybe u require that book in next class
2)the person doesn't use the book in justful manner like wasting the money of the book for selfish motives
3)Maybe ur mom has promised to give it to someone among her friends
Yeah donating books is a very good and noble a job... but u must take care that u give books to only the needy ones who makes the fullest and justful use of knowledge... but 1st keep in mind the ur future needs and even ask ur parents and teachers about it before just handing over the book!!
Hope it helps u!!:)☺