Probably a grand worth of paper and ink. Worth every penny.
In the mid 1990s, I taught myself Perl so I could build more powerful websites. After false starts with books from other publishers, I stumbled on O'Reilly's "Learning Perl^1^^" by Randal L. Schwartz and Tom Christiansen. That's when my journey hacking code truly began. Ever since, whenever I've needed to pick up a new computer skill set, my first step is to see if O'Reilly has a book on the topic.
Most of these books are old and will be retired to the library soon. If I consolidated all the other O'Reilly titles scattered around my home and office, I'd have at least another shelf's worth. That's not counting titles I've given away over the years. There would also be significantly more if hard copies of all eBooks and Safari Books Online^2^^ texts were included. All that's to say that I'm a fan of the O'Reilly publishing house. If you need to pick up a tech skill, start with them.