I highly recommend the online archives of the American Rhododendron Society. There might be a bit of outdated advice here and there, but overall I think I've learned the most from reading those than any other single source. Another great source for me was visiting Rarefind and getting the golf cart tour with Hank or Ron but that option is no longer as Hank is deceased and Ron has left. I expect them to continue to carry wholesale plants from Briggs et al.* but not really be a center of east coast rhodie breeding and selecting as they once were. The display gardens are definitely not what they used to be. If you like what you see in the mountains around you you would have been amazed by what you could have seen there back in the day. I wasn't even that into rhodies when I went up there in 2006 but that got me hooked. I didn't know you could have such gardens outside a maritime climate.

http://scholar.lib.vt.edu/ejournals/JARS/v35n2/v35n2-ring.htm

There are books with, again, generally good advice...but if you take the Coxes for example, probably the world's most famous rhodie family these days, their advice might be better in a maritime climate than in yours. That's why I linked to the particular article I did. I like the Cox's encyclopedia of hybrids but it's very UK-centric, and I wouldn't consider it a beginner book. You need some experience growing them first.

You can download old pdfs of Rarefind's catalogs, they do have some useful bits of info. Like Hank's famous description of the 10 things for success with rhodies, which is 1) drainage 2) drainage 3) drainage 4) drainage LOL.

* - and for a high price, too. Locally, Groff's plant farm has a limited selection of Briggs rhodies for $12.95 for a big 2 or 3 gallon pot, something Rarefind charges at least twice as much for.

Here's the latest:

http://www.rarefindnursery.com/media/pdf/RareFind-Nursery_2016-catalog-web.pdf

Hank's 'Criteria for Success with Rhododendrons' is on page 8.

It was written many years ago now, but David Leach's book, Rhododendrons of the World is still a great introduction to the vast array of rhododendron species, their biology and culture. David's recommendation of the Cox books is excellent advice. Kenneth Cox's Rhododendrons and Azaleas, A Color Guide is a monumental work with photos of hundreds and hundreds of hybrids and species. Its focus is less on culture than in giving a sense of what hybridizers have achieved in the years since the Leach book was published. Harold Greer's Handbook of Available Rhododendrons is somewhat dated but remains a valuable reference. Finally, don't overlook the catalogs, print or digital, put out by some of the retail rhododendron nurseries. Those from Singing Tree Gardens (online only) and Whitney Gardens (online and print) contain lots of information about growing in general and conditions best for different varieties.

For some reason I left out Edward Reilly's book, Success With Rhododendrons and Azaleas. This is a very comprehensive book encompassing everything from cultivation to propagation. It is more recent than the Leach book and contains a wealth of information in an accessible, but scientifically rigorous form. If you buy or borrow only one book, this is probably the best one to get.