Andy,
Pre-war (Coleman era) Flyer O Gauge is a particularly rewarding niche. It's remarkably neglected - most Flyer nuts are postwar S - and so prices can be surprisingly favorable. I also find the designs to be refreshingly idiosynchratic - that is to say, totally cool.
As an offshoot to my prewar standard gauge layout, I got side tracked onto Flyer prewar accessories, which is an ongoing delightful adventure. The early cast iron street lamps and signals, the later signals and crossing gates, the wide (and initially confusing) array of stations, and the Flyer concept of accessory "sets" (various combinations of stations and signals mounted together on one base)... very nifty stuff.
And of course the trains themselves are marvelous, with great lithography (flying locomotives and lightning bolts, you gotta love it!)... There are some great freight that are every bit as collectible - and as much a challenge - as Ives herald cars. Morris box cars anyone?
Steeple Cabs, Hummers, The Royal Blue, the Bluebird, Oriental Limited... it's a whole unique universe..
There are some great Flyer books out there. I agree with Bob that the Greenberg 1997 reference is essential if you're going to try to make some sense out of it all.
Enjoy!!