The three resources I mostly use are:
This is a listserve mostly used by registrars and collection managers that allows an open forum for questions and answers to be shared. It also has some job listings.
2. Museum Registration Methods 5th Edition. By Rebecca A. Buck
This book is the holy grail for registrars. It has all the basic principles a registrar might need.
Here is a nice blog I sometimes look at that is about the use of technology in museums and some other interesting things. A great post that I found interesting was the one about QR.