I have been looking to get into microprogramer like devices for quite a while now to do small control projects. I have a degree in Mech Eng and have taken classes in Electronics (analog) and some limited programming. I am looking for a good book or some websites that can get me started. I am not totally oblivious to the subject but have a couple of questions already.

Does it matter what language they are programmed in because it runs through a compiler into assembly or are they all programmed in C or C+
I see different types like PLC, PIC, AVR, what are the differences and are there others?
Are they all burned with the same devices or is it manufacturer specific.
Some setups look like more complete boards than just chips, what are the differences/advantages?
Thanks