I don't think I would have tackled this project if it wasn't for the wealth of information available on the web. Here are some good source of information on droid building.
Astromech.net - The official web site of the R2 Builder's Club. If you are going to do a build this is the first site you should sign up for. The message board has a wealth of great information and a lot of active member that can answer any questions you might have. There is a community of people who make droid parts so there are a couple forums on this site that announce when people are going to do runs of the parts. Also be sure to check out the Builders Logs forum which has build logs like this one.
Astromech.net now also hosts all the official blueprints that were alson on the Yahoo Group. You can find these files here.
R2-D2 Builders Club Yahoo Group - You will want to sign up for this group if for no other reason then to access the files area. Here you will find an archive of "official" blueprints as well as tons of other files to help with your build.
Droid Wiki - Part of the Astromech.net site, this has some nicely organized information on droid building. If you are starting out I highly recommend reading the Introduction, Beginners Guide, and the CSR Overview.
Besides the builders logs on the Astromech.net forum, you will find a lot of blogs that show people’s builds. I find my self referring to James Feurtado’s blog a lot because he built an R5 and he did a lot of it from scratch using wood so it’s a close match to how I plan to build. I also like a lot of the techniques found in this blog.
These are just a few of the resources I have been using so far. I will post more as I go along.