It is like Humble Bundle but at larger scale. It depends if you were already planning to buy a packthub book before or not.https://www.reddit.com/r/deals/comments/dkvx6j/100_packthub_ebooks_300500hrs_of_video_courses/
It is pretty much all the books that one can legally acquire oneself without going through the store. Pakthub has partnered with several sites to offer their content. It is a bit like O'Reilly/Safari Online is taking most of the books from PacktHub and other publisher to have in the library. While PacktHub is underselling their books with other partners to get people to buy their stuff. You can look if there is a book you had on your wishlist. There is a platform that decided to sell all of its video courses along with some of the 100 books that PacktHub gave away to partners in order to generate sales.
The 100 books also include several PacktHub videos. It is all e-books.