If you really enjoy them, buy them or find a way to donate to the author(s).
If you want to support the charities, donate directly.
Humble Bundle doesn't make sense to books. It only makes sense for a few games where you can really get a good offer, for lets say, a game and the main DLCs, but such promotional campaigns rarely happen.
And given that the purpose of HB is to get money donated to charity, it's not an efficient way of achieving that because they get a very small fraction of what we pay.
Hence why I said you'd better off pirating, evaluate the book value, and if needed buy the book to support the authors.
And if charity is also important because they earn more money.
Suggesting you should pirate these books because you think they're outdated is just 'splaining away being too cheap/frugal to pay for things that you want.