The convenient websites are: 1. National Library Reference Union: There are abundant stacks, but only 50 pages can be downloaded a day. After searching for the book you want, click "document delivery: library document delivery" in the right column, fill in the download page range (within 50 pages) after logging in, fill in the email address, and it will be sent to this email address after a while.

2.Kumo Search for Books: You can find the books you want by searching for related keywords.

3. Library Genesis: This is a foreign language website, but Chinese books are also super rich, and traditional textbooks can even find the original directly. After searching for the book you want to read, click Mirror to enter a new page, and then click Get to download it.