..if you are interested in learning more about the nitty gritty of ARM processors, I can recommend "The Definitive Guide to ARM Cortex-M3 and Cortex-M4 Processors" by Joseph Yiu - I've found it very helpful.
I have a couple of the Joseph Yiu books, later ones covering more material.
When I moved from the ARM7/ARM9 parts I used the First Edition Definitive Cortex-M3 book to get my bearings.
As others have indicated having a good foundation of C on a PC is really good, easier to test ideas and algorithms, and also Micro-Controllers Architecture / MCUs in general, these things are all more alike than different. I personally feel understanding the plumbing is more important than specific makes/models, as these change far too rapidly.
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".... C on a PC is really good, easier to test ideas and algorithms" - yes, I've been using Turbo Pascal for years to test various filters, FFT components, generate lookup tables etc.
Further on the Yiu books. I looked at the Programming manuals for the STM32 chips - they have most of the information required, but not in a form that is as easy to read and understand as the book.