I am a sucker for an obscure WW2 army, and this Osprey MAA by Nigel Thomas certainly meets that criteria.
The author starts with the pre-war Czechoslovak Army, fatally undermined by the 1938 Munich agreement, stripping them of the Sudeten fortifications. They benefited from an advanced armaments industry, equipping the army with modern artillery, armoured cars and tanks. It was never tested other than a limited resistance to Hungary's invasion of Ruthenia. However, the Germans made extensive use of their equipment throughout the war. All the uniform details are here if you want to give this army a go for what-if scenarios.
After the fall of Czechoslovakia in 1938-39, elements of the armed forces fought in Poland, and others emigrated to France. In France, they joined the Foreign Legion or the French Army. There was an 11,405-strong 1st Czechoslovak Inf Division, commanded by Brig-Gen Jan Kratochvíl. After the fall of France, some 3,500 troops evacuated to Britain.
These forces and others served with the British Army in the Middle East, the Far East and NW Europe, mainly in British kit with a Czech flag on the helmet and shoulder flashes. There are the usual excellent colour plates. Aircrew served in the RAF, forming three fighter squadrons, a bomber, and a night fighter squadron.
Troops interned in the Soviet Union formed a battalion in the Red Army, growing to a corps of 16,000 men by 1944. They attempted to support the Slovak uprising in August 1944 but suffered heavy casualties at the Dukla Pass.
On the home front, various internal security units were created, although the Germans didn't trust them and were limited to guard duties. Resistance units started almost immediately and were supported by SOE. This included the famous Heydrich assassination and the destruction of the villages of Lidice and Ležák. The Russians and Americans converged on Prague, which was liberated by the Soviets in May 1945, supported by an uprising.
It wouldn't take much to add Czech units to your Allied or Soviet armies. I suspect the internal security and other exotic units might be a project too far for most.