This book I find very useful for identifying wild flowers. It has good, clear photos inside.
It's too large to be used as a field guide, but then I find most field guides too small to be of much use anyhow.
It's by Rae Spencer-Jones and Sarah Cuttle, and published by Kyle Cathie ltd, in association with Plant life.
I was lucky enough to find this copy in a charity shop, but new its £14.99.