Gunny,
If you're looking at US Classics, probably the most important and useful book is either the 2-volume or 3-volume set written by Lester Brookman "The Nineteenth Century Postage Stamps of the United States". There are several offerings right now on
ebay. Each issue is described with all variations, usages, with detailed diagrams-- only problem is some of the old references are in B&W, so it doesn't help much with color. If you're patient, you'll find reasonably priced copies out there.
Covers are a completely different story. The Brookman book(s) have some information on covers, but you might want to understand all of the issues first, in case you want to consider specializing.
Of course it all comes down to if you want to fill holes, accumulate everything you can get your hands on, or specialize in 1 or several issues.
Either way, good luck.......Ray