Plus there are a lot of books, where one version is a high quality scan, but no OCR, and the other is OCRed scan (with a bunch of errors, but searching works 80% of the time) and horrible scan quality.
Also, some books included appendices, that are scanned in some versions but not in others, plus large posters, that are shrunk to a4 size in one version, split onto multiple a4 pages in another, and one huge page in a third version.
Then there are zips of books, containing 1 pdf + eg. example code, libraries, etc (eg progrmaming books).
Also, some books included appendices, that are scanned in some versions but not in others, plus large posters, that are shrunk to a4 size in one version, split onto multiple a4 pages in another, and one huge page in a third version.
Then there are zips of books, containing 1 pdf + eg. example code, libraries, etc (eg progrmaming books).