Hi Mike,
I guess it depends on how far you want to 'get into them' - If you just want to get an introduction to the group, this book is fine but you'll quickly find it frustrating as it only covers a handful of species:
http://www.amazon.com/Myxomycetes-A-Handbook-Slime-Molds/dp/0881924393
This next one I don't own (yet!) but I have heard that it contains some beautiful images and is probably the most up-to-date. t's pretty pricey and I imagine that at least part of the text is in French, but at least the keys are in English:
http://www.summerfieldbooks.com/les-myxomyc%E8tes.~3341
This series contains some stunning drawings and photos and is split into three taxonomic groups (for the more discerning myxomycologist!). One downside is that all the text is in German (and another is the price!):
http://www.nhbs.com/series/52021/die-myxomyceten
This one is considered the 'definitive' guide to British Myxos. It's out of print, but you can sometimes find a cheap copy if you keep an eye out on the auction sites. There are no colour photos and I suspect that my three year old daughter could come up with better illustrations, but the keys are fairly straight forward and it's still (more or less) up-to-date:
http://www.amazon.co.uk/The-Myxomycetes-Britain-Ireland-Identification/dp/0855462515
There's also a lot of good stuff on the internet - I recommend downloading (for free!) Listers' immense monograph. It's more than a century old, so parts are out-of-date (but you can always cross reference names here:
http://www.indexfungorum.org/Names/Names.asp).
Personally, I printed the whole thing (it takes a while) and regularly use the amazingly detailed illustrations as a reference (it's worth noting that if you want to print the illustrations that they are on
every other page - They start at around page 75). It took me quite a while to get used to the key as it's quite different to a standardised modern one, but I think it works O.K after several practice runs:
http://www.biodiversitylibrary.org/item/61303#page/382/mode/1up
Another site that I regularly use for reference is this one:
http://www.discoverlife.org/mp/20q
If you have a 'click around' you'll find some lovely photos, as well as micro pics and the odd Scanning Electron Microscope photo. There are full descriptions too!
Hope that helps....
I know all the terminology takes a while to get used to, as do the cryptic lifestyles and many other aspects of the group, but if you stick with them they are very rewarding (and suprising!) organisms. You can always post your findings here and I'll try you out to the best of my ability (I know my way around a few of the genera as my username might suggest!).
I can't remember if you're one of the regulars here that have access to a microscope, but I'm always willing to take a look at a specimen if you don't mind paying the postage (this offer is open to any members of the forum - just PM me).
Cheers,
Nick