One of my many books on Poker is "The Mammoth book of poker" which dedicates whole chapters on the different formats of the game, from Holdem, Omaha, Razz, Stud and many many more, it is the best investment I ever made.
Books by professional Players like Brunson, Helmut, and Greenstein are pretty good but generally only give an overview of the game from their perspective and most of the time is spent on their Poker history and how they approach different
hands.
But overall the best lessons I learned was from Cardschat "How to be a Winning player in 30 days" When I first started the course I thought that I knew everything that came up, but! there is nothing wrong in revisiting the course and going over it time and again, after all isn't that what students do when preparing for an exam. Study, Study and more Study.
Poker evolves over time and changes with every game that's played the only way to keep up with the changes is to play and learn from your mistakes, through hand analysis and study. Just like driving a car is nothing like driving 30 years ago, the highway code changes and new road signs come out, so you have to study over and over again to keep up with the new rules of the road.
Experience is the best method of learning anything, after all you can read all the books in the World on Brain Surgery, but until you get someone under your knife, you cannot call yourself a Brain Surgeon.
When the fun Stops, STOP!
(or take up Surgery)