The 32-bit ARM microcontrollers are in a totally different league to the Atmel and PIC devices you have had experience with. You should expect there to be a steeper learning curve with the significantly more powerful, capable and complex ARM devices.
I would recommend that you start off with one of the smaller ARM Cortex-M3 systems aimed at beginners (e.g. an NXP LPC176x mbed board or LPCXpresso board). I would not recommend C if you had no previous programming experience but as you already have then you know what to expect there. You do NOT have to use CMSIS if you do not want to. The book I recommend is Joseph Yiu’s “The Definitive Guide to the ARM Cortex-M3”. You should also download the User Manual and Datasheet for your chosen microcontroller from the manufacturer’s website.
Good Luck!