Now that the #COVID19 is shutting down the entire diving Planet, here are a list of few books recommended to extend your theoretical knowledge:
Staying Alive by Steve Lewis
The Six Skills and Other Discussions: Creative Solutions for Technical Divers by Steve Lewis
Advanced Wreck Diving Handbook By Gary Gentile
Under pressure – Diving deeper with Human Factors by Gareth Lock
For any diving technical training done together, you’ll get a free “Human Factors Skills in Diving – Micro-class“, compulsory to be completed before ending the theory and practical skills.
Deep Into Deco by Asser Salama
Ocean Gladiator by Mark Ellyatt
Death in number two shaft by Steve Lewis
A compiling of various scuba diving materials can be found here:
Other internet sources: Blue PO2 Diving (scuba & technical), PADI Blog, Good Reads, Dipndive etc.