Cool series of trips. You could travel the entire western US, visiting groupings of lookouts each summer.
We had a pretty extensive series of lookout towers (historically) in our local National Forest (Los Padres). Some have interesting histories, such as a few that were repurposed as lookouts for Japanese subs and fighter planes during WWII. Sadly we've lost most of them over time to various events or just general neglect... some burned up in fires (after they were abandoned), a few were purposely dismantled, etc. A lot of the old lookout sties here just have the footings left, or maybe the metal frame, but nothing else. We've got a few that actually still have the lookout structure on the top... in fact we just lost one in the last month or so; looks like it blew down in a recent storm.
Anyway pretty sure they're all reachable by bike. They're interesting to visit and generally have impressive views, but ours are nothing like the ones you've been visiting. The ones you've gone to so far look like a good time!
I'll give a +1 to Eugene's rec for "Fire Season." It was a quick, albeit interesting read.