Apart from a book, experimentation should go along. Also note that with experience, skill, changing circumstances, changing bodies, the goal posts shift.
If you couple breathing to self hypnosis you'll have a very profound and powerful effect.
I had few times a very extraordinary experience, being in total
flow and that made the dives very memorable. These days I train myself to fall in flow.
[ame=http://www.youtube.com/watch?v=ZqZbF546JNo]Frog Flow - YouTube[/ame]
You can practice many freediving aspects on land, but in the water they feel the best

Concentration, Qi Gong, flexibilty, focus, learning how to think, mental mind control, mental body control, diet, rhythm, etc.