umm, you can find tons of stuff at
www.arm.com, its more generalized about the arm processors, but the asm is there. Plus if you check out the GBA dev sites, you can learn a lot of ARM. And from what ive read about the GBA it uses a sort of dual mode processor, There is (16-bit)Thumb Mode and (32-bit)Arm mode. i would assume arm asm is arm asm. Plus intel has documents on it.
And its not really programming into the PocketPC API, but coding functions, such as an emulator. emulating the opcodes would be faster in arm asm, would it not??