Here are my recommendations, not in any order.. I'd consider these 'must haves' and can be considered to be 'the classics'.
Cocker Spaniel: An Owner's Guide - Jane Simmonds
The Perfect Puppy - Gwen Bailey
Total Recall - Pippa Mattinson
There are millions of training books of varying usefulness (ask me how I know!). I found those by Lez Graham to have great advice with the least waffle and the most engagingly written - The Pet Gundog and The Advanced Pet Gundog. Training Spaniels by Joe Irving is also a good read.
Though not cheap, I found all of these to be well worth the investment.