I do not regard myself as a good in ARM (specially in Cortex M core) but I would recommend a book: The Definitive Guide to the ARM Cortex-M3, Second Edition by ARM expert Joseph Yiu.
http://www.amazon.com/Definitive-Guide-ARM-Cortex-M3-Second/dp/185617963X.
I purchased it few weeks ago but to be honest, I've been busy these days to go through it. Please, read the Amazon.com comments and see. regards!