I have found this book very useful for learning about my local trees. There are some useful keys for identifying for example Southern Beeches, Limes and Cockspur Thorns. However, don't expect keys for all the trees you find as some are difficult for example the Flowering Cherries and Rowans (of which there are many hundreds of varieties).
This is a small book suitable for taking out into the park and therefore you soon end up wanting more. More species, more pictures, more info - more of everything in fact. But that is to the author's credit as by now he has developed your interest in the subject.
As an introduction to trees growing in Britain and found in our streets and parks I can recommend this as an excellent starting point. Despite having been written thirty years ago I can understand why others regard it highly.