Which should I put?

Answered

If you have the space for it, you can certainly list all 30. It also really depends on the prompt - are they asking for all the books you read, or a selection of them? Columbia's list questions, for example, have a 150-word limit, and they only want to hear about the works you "enjoyed most."

I'd also ask yourself what you want to convey with your list. Do you want to convey the volume of your reading? Your interest in reading memoirs, social psychology books, or 20th century women's lit? If your list has to be pared down, think about the kind of "image" you want to curate that supports your Applications Theme.

I will also add that the drawback of listing more than 10-15 books will make your list harder to read. It's more likely that the admissions officer will appreciate a shorter, more curated list (unless the prompt asks for all the books you read). Of course, not listing all of the books you read will also not show your volume of reading. There are pros and cons to weigh here.

Hope this helps, and let me know if you have more questions!