What was beethoven thinking ?

how did he compose a such a wonderful piece like moonlight sonata, i can't get over that song , how exactly does someone compose clical music, sometimes i can think of a melody but that is it, the longest i ever did was about 15 seconds, is there any books or some websites that can provide information on composing music ? can someone tell me the essential(i am not sure if that's the right word) about composing clical music