Superstar reader is a software that needs to pay money every year. There are many books in it, including a large number of professional books and periodicals, which can be downloaded after paying the money.
E-book alliance and e-book bar download e-books for free.
Baidu Library downloads documents with integral.
Douding Com is better than Baidu library.
The above can be found by Baidu search. You can also click on the friendship link provided by the above website.
All I gave were e-books. If you want to buy a physical book, go to the book wholesale market and get a 15% discount.