In addition to ADF & OBOD, which are definitely two of the largest organizations, there is the Henge of Keltria an org that is an offshoot of ADF.

And for some good articles concerning Celtic polytheism - http://www.imbas.org/
It's a now inactive org, and it's geared towards Celtic reconstructionist crowds, but anyone seriously interested in druidry would do well to read some of the essays.

ETA: the only neo-druidry book I've read entirely is Mysteries of Druidry
It's a pretty good book, I felt like it's a good basic introduction into neo-druidry.