For Russian/central Asia/Chinese conflicts in 1918-late 1920's which is what the Copplestone stuff is aimed at, some select reading:
Setting the East Ablaze (Hopkirk), general background and a MUST HAVE
The Spy Who Disappeared (Teague-Jones, ed Hopkirk) Caucasus and Trans Caspian adventures
White Terror – Warlords on the TranSiberian (Bisher), Siberia
The Bloody White Baron (Palmer), mad Russian Baron things he's a reincarnation of Genghis Khan, conquers Mongolia
Sand Against the Wind (Tuchman), Stilwel and the American experience in China 1911-45
Beasts, Men and Gods (Ossendowski)
Hunted through Central Asia (Nazaroff)
Dragon Hunter (Gallenkamp), Roy Chapman Andrews' archaeological expeditions in Mongolia and China in the 1920's – think Indiana Jones
Then there is the usual Ospreys, the Red and White Army books are essential. The armoured vehicles give more info and there's the Czech Army book as well. More specialised books based on the Osprey style are available from militaryhistorybooks.com
At this point I'll mention Mark Plants "Pygmy Wars" website which is a mine of information.
Then you also have the Ango-Afghan war you can do as well.
For warlord era China, others have more experience and suggestion, but the Osprey Chinese Army books give you a starting point, as does Stilwel's experiences.
Bob Murch's Pulp Adventure figures are very useful for this location, and in general for providing Chinese and Adventurer types. They're compatible with Copplestone.
Brigade also do loads of figures that are suitable for RCW/BoB and they are nice and compatible with Copplestone.
Other inter-War periods are:
- the Chaco war, Bolivia vs Paraguay
- Spanish Civil War
- A Very British Civil War (totally made up)
As for games systems:
Contemptible Little Armies, includes army lists for BoB
Setting the East Ablaze (yes that's a rule set, can't think where they got the name from)
A World Ablaze (from Osprey)
Various WW1 era rulesets are also used, I haven't ever played these, and some late colonial rulests are also used (ditto)
A mate of mine is bashing out a variant of "Went the Day Well?" called "Went the Revolution Well?" and its going to get its first big outing Sat 31st at a big game I'm organising in Herefordshire (UK).
HTH
Giles