As you know I've just started ringing and one of most important things to be able to do is age and sex birds correctly. In order to do this with a degree of confidence you need a copy of "the ringers' bible" - Identification Guide to European Passerines by Lars Svennson.
I know it doesn't look much, but this book has details of all european passerines or perching birds which includes over 200 species! Obviously I won't be able to learn them all at once, but then I don't expect to catch many Cretzschmar's Buntings or Bimaculated Larks in Chesterfield. Perhaps I'll concentrate on species such as Blue and Great Tit to start with!