Hey Shades of Green,
are you looking at buying actual books only?
Asking, because the PDFs of the five Core Rulebooks (that is, Street Magic, Arsenal, Augmentation, Unwired, and Runner's Companion) are only $12 each in PDF (
www.battlecorps.com - which is the official online store for CGL; you have to look closely, though, there are some older edition books mixed into the lists also). The SR4A book (20th Anniversary Rulebook, the Main Rulebook in its latest incarnation, which changes quite a few things under the hood, and is a very nice book overall) is only $15 in PDF.
I would definitely recommend getting the full set here, eventually, as they all integrate together. Though, Unwired is the one that you could skip the easiest (using only the basic Matrix rules, which are quite complex already, from the Main Rulebook -- if those seem to be lacking something for you, then you might want to take a closer look at Unwired).
Street Magic and Arsenal are pretty much must-haves. Augmentation and Runner's Companion are close.
Those are the first four books I would get, when I only had the main SR4 book, yet.
Running Wild, if you like having a larger amount of Critters to choose from, makes sense. I'm not using it much, though. It's not like D&D where you really need a variety of monsters as opposition. Most of the opponents in SR are metahumans after all.
Other than that, I think getting some setting books will be the most useful.
Seattle 2072 and/or Runner Havens for example. Both are good books, especially Seattle 2072 (if you want to use the iconic SR setting, that is).
If you are not a SR history buff, the Sixth World Almanac might also be worth a look. It's a bit shallow (i.e. not very detailed, especially when it comes to the geographic parts), though it does offer a good overview.
From future releases (coming soon), Attitude (Sixth World culture) is a nice addition. Runner's Toolkit might be useful as well.
Bye
Thanee