If you want to be better able to identify birds in the field, I recommend this book.
The subtitle is “How to identify birds, using the clues in feathers, habitats, behaviors, and sounds.” Not a field guide to species identification, it’s a field guide to how to learn about birds.
There are sections on how to compare closely related species (is that a hairy or a downy woodpecker?) and how backgrounds and light changes can influence the way you see the bird.
David Sibley recommends imitating bird sounds, which makes me feel better about wandering around whistling bird noises all the time. He even draws some bird songs, a very useful field technique.
If you want to talk about uppertail coverts, auriculars, and juvenal plumage, this is the book for you. But don’t be scared off by the vocabulary—this is the book to help you learn it, with Sibley’s exquisite sketches to illustrate every concept.
The chapter on feathers is thorough, and a great help for matching individual feathers to the part of the bird they came from, plus being able to tell the age of the feather and the bird. The section on molting gives great hints about understanding feather wear—it is very helpful to be able to pick up a feather and tell if the bird was stressed while it was growing the feather!
An excellent companion to your identification guide.