The system occupies a certain space, generally less than 3G. As for the size of the book, it's hard to say. After all, it's not txt. E-books in Central Asia contain fonts and pictures, and the files may be very large. It is best to connect the kindle to the computer and look at the files in the documents directory. And there are dictionaries and screenshots, which also take up a certain amount of space.
How to find free book resources on Kindle? Enter the Kindle e-bookstore and select "free e-books" in the "Recommended for you" column above, and there will be a big wave immediately.