I've heard that the second edition rules have some glaring errors and omissions, almost like they released it before it was finished, you can find this easily as a PDF on-line as
GW release it as such for free download, although I'm not sure where would be best to get it from now as it is no longer available through their site:
The main rule book that my friends and I use is the compiled Necromunda hard-back that combines the original rules and the outlander rules into one volume. The book is ~312 pages and can sometimes be found on eBay quite cheap (I picked mine up in mint condition for £20.00):
The guys over at yaktribe (formerly Yakromunda) maintain a "living rulebook" which gets updated regularly. They also have many resources with alternative rules, roster management, etc. Well worth a look if you're curious:
http://gaming.yaktribe.org/
I've got quite a few links for various PDF copies of rules, send me a
PM for them if you're interested.