I have built dozens and dozens of PCs... (actually, I'm quite sure that it's over a hundred.) Modern PCs practically assemble themselves! They are very easy to put together.
If you are putting together a bottom-tier PC, the price savings isn't
huge, but if you start to go slightly higher-end, you can start to save a lot of money.
One of the best places to buy components is NewEgg and
this morning's email ad had a pretty good deal for a complete system... $310 after rebate for a case, motherboard, fast (but only dual-core) processor, 4 gb of RAM, 750g drive, DVD burner, and a decent motherboard with HDMI. Saves about $100 over buying the parts individually.
I don't build as many as I used to so I'm not up on the absolute best choices; what I do lately is look for the stuff on NewEgg that is 1) highly-rated by customers, 2) has lots of reviews (so is a popular choice), and 3) is cheap, like me.

I tend to go with Athlon processors although I understand that the new Intel i7 is supposed to be good - but still, AMD generally gives you as much or more performance for a good bit less money.
I would consider 4gb the minimum for you since you'll be working with large image files. 8gb wouldn't hurt, but as long as you have four memory slots on the motherboard, you can add the second 4gb if you feel like you need more speed.
If you won't be playing games, you can generally get away with the cheapest video card you can find. Even the cheap ones have half-way decent 3D now, usually enough to run Win7's "Aero" interface smoothly.
Absolutely, positively, Win7 64-bit is the way to go. After using it for a while, XP feels downright primitive (though still very functional.) Home Premium is probably the most sensible version.
You may be able to find some books but the technology changes so quickly that it's hard to keep up. Some of the big sites for PC building include Tom's Hardware, Anandtech, and several others. You can probably find some basic how-tos on eBay, too.
There are no hidden costs as long as you get all the components. Make sure to buy the OEM version of Windows 7; you can legally do that as long as you're buying the hardware to put it on and it's a good bit cheaper than the retail version.
The actual assembly is pretty easy. Install power supply in case if it's not there already... screw motherboard, hard drive, and DVD burner into case. Drop CPU into socket, mount CPU fan (the fan can sometimes be the hardest part of the whole thing!)... Put memory into its slots... connect power to motherboard and the drives... connect the front USB and power/reset/LED connections to motherboards (easy as long as you follow the motherboard's instructions)... install SATA cables to hard drive and DVD... install any add-in cards like video... then boot it up and install Win7 (which is trivial!)
USB3 is starting to appear now, you may want to look for that if you want to keep on the bleeding edge. You could also grab a Bluray reader/DVD burner if you want to watch movies occasionally... or even a Bluray burner, I think they're under $150 now. Not bad, but DVD burners are usually $25 or less! You'll probably want to put in a card reader, too.
Good luck! Let us know how it goes!