This is a very simple script for retrieving a list of book ID's from OReilly's online book library: https://www.oreilly.com/online-learning/
Once you have all the book IDs, you can use the program in the repository below to download them as an .epub file.
https://github.com/lorenzodifuccia/safaribooks
For your convenience, the book IDs for selected topics are given in the accompanying .txt files.