It's amazing that those who have the most reasons to withhold knowledge are the most generous givers.
Like Ivo Welch's amazing Corporate Finance textbook which I benefited immensely from without paying a kobo. Helped me break into the Finance world, even as a consultant.
Also the book that got me started on Python and taught me more than all the other books I bought: Automate The Boring Stuff With Python by Al Sweigart
Even the book I am currently using to learn Data Structure and Algorithms: Problem Solving With Algorithms and Data Structures using Python by Brad Miller and David Ranum.
And we have got one of our own, Ade Oshineye, repping us nicely. He is a senior engineer with Google and has co-authored a highly rated book that is also accessible freely online: Apprenticeship Patterns
But the father of them all is that which MIT has done: making virtually all their course material available freely online for anyone to learn from without paying a kobo. Topnotch quality material from the world's highest ranking University.
You can access it all at: MIT (Massachusetts Institute of Technology) Open Courseware. You won't get a certificate at the end. Knowledge is your reward.