In modern times, with e-books, I think you don't have to spend money on purchasing, because almost everything can be found freely available on the web.
Absolutely have. Most I have read are the stories of what made each successful and how they play. One fella that really does help with tactics is Sklansky. He's good.
I think that going to a training session with a good coach will be much better and more rewarding than reading a book that was written several years ago.
Modern Poker Theory by Michael Acevado is probably the best theory book I have read. Matthew Janda books are also good reads. Many other books I feel are outdated and fail to integrate new learnings from solvers.