I have been an AD and imaging specialist for about 25 years, and have also taught imaging techniques & pshop to other professionals (designers & photographers). I USED to suggest Hayes/Crumpler Photoshop Artistry, god knows what version is out now. The book is especially useful if you are coming from a photography side, but I have used it for designers as well.
All that being said, despite being a die-hard Pshop fan, the bloom has faded on the rose regarding Adobe, and Photoshop. Despite annual "versions", not much has really functionally changed since PS6 (IMO), except the pricing schemes (and I mean schemes) to bilk the professionals with badly opted upgrade paths. The cost has spirialed beyond reason, yet the functionality has not. Sort of like making blenders and just keep adding "speeds" as bling over substance. Furthermore Adobe has been ridiculous with their slow pace of development on the Mac side, which is frustrating since they essentially started as products for professionals who used macs. Lots to be attacked there about, and I really don't care to go there...
If you are looking for software understanding... check out Lynda.com. They have a number of free tutorials, and if you subscribe for a month or a quarter, the cost is not too bad. The amount of aps they cover is pretty huge. Be careful of many of the seminars, as many are a waste of time. Talk to others who have taken the courses before you leap. Apple also offers some basic courses covering iPhoto and Aperture, many of these are free.
After spending most of my career working with pshop, as of late I have slowly been moving away. Stuff that involves type, and or layers still goes through pshop... but other than that I use Aperture. A year ago I embraced the NIK plug-ins for Aperture from Nikon... WOW! It is amazing what can be done, and the control over your imaging that it brings. The logic is unlike pshop, but very easy to grasp quickly... as is Aperture itself. I can teach someone to use both on a basic level MUCH faster than explaining imaging basics in pshop. The NIK stuff is available as a suite, and the total price is only a bit more than just one of the filters/plug-ins (look for specials too). Aperture is dirt cheap IMO as well, and you might be able to get away without having pshop as well (unless you will need to merge/render type on your pics, and don't want to do that in Quark/InDesign... or if you need to work with a lot of layers, or need to do compositing work... like swapping someones head etc.). Nikon has a great website with a lot of free tutorials as well. The suite runs $299, and Aperture is now $199 ($155 at Amazon).
I work on thousands on images each year, and BOTH are a joy to work with, and MUCH MUCH faster than working in pshop. Nikons tutorials are here:
http://www.niksoftware.com/learnmore/usa/entry.php
Both aps have free trials to try, and I would suggest trying both before you made any decision. The best way to learn of course is by doing...
and calibrate your monitor as well. I use a hardware kit called Sypder2, and there are tons of different ones out there for $99-199.
best of luck to you,
michael