There's a whole new world out there of strange alternatives to our usual philatelic universe! Revenues, cinderellas, poster stamps etc etc.
Some are catalogued in depth, others need some real research. Barefoot do an excellent range of catalogues, but there are still large areas not properly catalogued. The standard comprehensive work is Fournier, but the most recent catalogue is 1908!! Early collectors collected all sorts of stamps, including revenues, but over time most collectors have concentrated on the postal issues. Gibbons have just recently started rolling out listings of telegraph stamps, and there are specialised catalogues (often of just one part of the world) of railway stamps and other postal services.
All in all it's a fascinating field and one that constantly surprises. I'm listing revenues and cinderellas on one day each week so gradually my stocks should build up, though they do tend to sell pretty quickly!
The pictures are of some of the stamps that have gone on this week.