You can borrow one ebook per month and keep it out as long as you want. However, you cannot borrow a second ebook until the first is returned.
My experience is there are millions of fiction ebooks available for borrowing which my wife uses. However, I have not seen serious, footnoted, history or biography ebooks available for loan. Those are the only ebooks I read.
I believe all NY Times 100 best sellers are available for loan.