I took a look at the Half-Life 2 source code in order to start making a mod or two which I have planned. It is a bit intimidating to look at at the moment though, since there are no real guides out there for the coding, they are all for map editing.
I noticed that Prima are coming out with a new guide in the next few months however, The Half-Life 2 Mod Maker's Guide, so I may pick that up to get some pointers. Even if I don't end up using it directly for HL2 modding, I'm sure there are some good general tips in there, and it is less $15.
Just one last note, I have been using Vonage (who use VOIP technology which lets you make phone calls using your Internet connection so you save money, especially on international calls!) as my telephone service for a while now and I can only say good things about them. If anyone out there is thinking of signing up, email me, (or use the form on the left of this page) an I can send you an e-coupon for a free month of service, whichever plan you choose.