Some very niche books, like books that are related to a game series for example. Also some university literature, scientific papers and history/culture books that are either hard to find or very expensive locally.

AA has managed to stay up for surprisingly long. I hope they will continue to do so. There is a large amount of documents that are hard to find anywhere else there, not to mention all the university books that would cost a a fortune to buy for a poor student…

EDIT: typo.

Notesnook should do it! It has a premium monthly subscription thing that gives you some extra functionality, but the free version does sync automatically between devices. I’ve been using it for a year now myself and have had no complaints.