Bob Nuckoll's book is absolutely a great resource for getting the basic architecture and concepts down. Wiring up lights, switches and stuff is relatively easy, even for a first-timer. As far as radios, transponders and PFD's (if you go that way), I'm a big fan of outsourcing to SteinAir or a similar supplier. Not only do they get the myriad pins and D-sub connections correct, they understand all the interconnections of the different components and how they communicate with each other. They also fire it all up on their bench to make sure everything is working correctly, all the software revision are correct, the configurations are properly done, etc. There's a lot to that.