Richards feels that modern fly line designs call for yet another revision and he recommends that this one be based upon length. It can all be a little too technical at times, but it is worth your time to read and re-read his examination. It is an excellent explanation of how to get the most out of your entire set-up, including leaders, and why it all does what it does. Lefty's Little Library books have been around for a few years and are available via mail order. If you've ever subscribed to a fly fishing magazine, you've probably already been showered with the direct mail campaign.
Advertisements still pop up from time to time as well.
You may find the books in shops, but the only sure way I know of to get them is to enroll, meaning you'll receive a book every other month or so, sight unseen. Some of the books are specific to types of fly fishing you may not care about. Some may totally miss the mark for you. In that case, you have to go through the hassle of returning the parcel. You'll probably put it off long enough to end up buying a book you don't really want.