Growing hardy palms (Leonard Holmes) - US-based site
Pacific Northwest Palm and Exotic Plant Society - another North American site, great pics of some amazing gardens in Seattle & Vancouver :-) And the North American Pacific Northwest has a fairly similar climate to north-western Europe...
http://snowpalm.dyndns.org/eng/ - hardy palms in Scandinavia!
http://www.france-palmier.com (en franšais) - good information on hardy palms for French-speakers :-)
Fous des palmiers - French palm society (aussi en franšais)
Discussion boards (public)
IPS Forums - discussion board of the International Palm Society
The Palm Centre - specialist supplier of exotics, based in London but mail order available. I have ordered from them a few times and plants are generally of very good quality. Carriage charges for these plants can be quite high, especially if your delivery has to cross water as well as land (this is not the fault of the nursery by the way).
Glenhirst cactus nursery (friendly service, quick delivery!)
Dicksonia direct. Finally an exotic nursery in Ireland!
Some good books
"Identifying Palms" by Martin Gibbons, 80 pages, published by the Apple Press and available from the Palm Centre (see link above). Price was GBP5.99 the last time I looked. Contains pictures and general info on many species.
"Palms throughout the world" by David L. Jones, 410 pages, published by Reed. Available from major bookshops and the Palm Centre. Now priced around GBP25. Contains information, descriptions and pics of many palm species.
"Architectural plants", Christine Shaw, Collins, 2003. Some great info here, although I think the author is a little cautious regarding the hardiness of some of the plants described.
"Palms won't grow here and other myths", David A. Franco, Timber Press, reprint 2003. This one goes the other way to the above if anything, some astonishing survival temperatures quoted here (the author is growing some of these plants in zone 6a - Ohio!)
"An encyclopaedia of cultivated palms" by Robert Lee Riffle and Paul Craft. I think this has become my favourite palm book! It has a fantastic amount of info, especially relating to hardiness, and hundreds of high-quality pictures. Sadly Mr. Riffle, one of the world's most knowledgeable and respected palm experts, died in 2006.
I can recommend these books as I have them all.