RTOS, I2C and SPI are separate things, so probably best to learn them one at a time, and order it so that the RTOS is last.
For uC/OS-II it's probably best to purchase the related book (it states in simple language how it works). However,
you should also read a generic book on RTOS too, so that you're aware that all things uC/OS-II are not necessarily
the only way to do things, nor the best way - remember that uC-OS-II was written at a time when memory was seriously
constrained - nowadays we have more memory and processing capability and more peripherals, and we may want to
exploit these in an OS with more capabilities. Anyway, that's just a personal opinion, I've not looked to see if
it has improved over the years, maybe it has.
For I2C, the best way to learn is to look at a Philips/NXP datasheet for an I2C i.c, and compare it to example code
found on the web, it is quite simple, and can be debugged easily with a scope.
For SPI, usually you don't need to do much, just follow the instructions in the manual for the particular microcontroller
that you're using, since usually many microcontrollers have a built-in peripheral to handle SPI.
Olimex make some low cost ARM boards. An example low cost ic is AT91SAM7S series.