Trees Of Britain & Europe by Gregor Aas and Andreas Riedmiller. Illustrated by Heinz Bognor. Translated and adapted by Martin Walters. 

I absolutely Love this little tree book! I think I found it while rummaging through a charity shop or some other literature treasure trove. 

Listing over 300 trees found in Britain and Europe, arranged by leaf shape. 
Containing 800 photos illustrating the whole tree as well as leaves and flowers. There is also a section for bark texture and colour and a useful guide to tree biology.