学 = study , it is a verb. 学习 = studying
I am studying = I am 学习
I study English = I 学 English.
念书 = reading , and sometimes 学习 and 念书 are the same mean
and 念书 not always do with books ,it also can show you are a student. 他还在念书 sometimes means "he is a student",it also can mean" He still is reading."