Mastering Mountain Bike Skills by Lopes is good for the rad skillz. Covers loads of stuff.
Friels training bible is good too if you invest the time in planning etc.
I quite liked the Time Crunched Cyclist book too. Don’t really use it much now but good for ideas.
Learnt a lot from reading the Swim Smooth book too, still rubbish at it, but not quite as bad as I was!
Edit: Reading Tom Danielson’s Core Advantage. Looks pretty good so far for road cycling specific core work that I can actually see myself managing to fit in. Yet to put it into practice though.
Double edit: I also got a lot of good info from Coggan and Hunter’s Training and Racing with a Power Meter. Helps make sense of the data and tools that analyse that data (e.g. TrainingPeaks.)