I've got an Excel spreadsheet for armor, aircraft and others. I print out the spreadsheet and use it as an index of my in action titles. I put the printed page in the front insert window of a large 3-ring binder.
Each 3-ring binders has heavy duty document protectors that I put each book inside. My armor books are in one binder, aircraft 1000-1059, aircraft 1060-1099, aircraft 1100-1149, aircraft 1150-1199 all in separate binders. Warships and troops are in a shared binder. Armor walk arounds and aircraft walk arounds are each in separate binders. Off brand armor books are in another binder.
Organization is the key to not buying duplicates.
I buy every armor title, but not at full price. I wait until Squadron has some sort of sale or if I see one on eBay for cheap. I first ran across the T-34 one at the Fort Knox Armor School book store and thought it was overpriced at $4.95 (in 1987). But as a new, single lieutenant, I had more money than I needed and bought it anyway. I had bought a 1/48 scale Bandai T-34/76 that had an interior and needed the book for reference.
I began my collection in earnest by buying them at the Stars and Stripes bookstore in Mannheim, Germany. Stars and Stripes carried them for a dollar off the cover price. Most of the ones I got from them still have a price tag over the pre-printed cover price.