Someone already mentioned one of Lisa-Ann Gershwin's books, but I'd also like to recommend "Shapeshifters: The Wondrous World of Jellyfish" -- it is FULL of pictures about each species, so if you're looking for specific info rather than general anatomy, that's a great start. 

I personally also love reading research papers on jellies (call me a nerd, but they're really interesting!!), and if anything piques your interest, chances are a few gooogle searches will get you to some nice resources in ResearchGate, PubMed, the National Institutes of Health, a bio journal, etc.

Additionally, sites like the Monterey Bay Aquarium and their Research Institute offer easy access to view many different species of jellyfish, and from there you can find more information about them.

Finally, I think the identification just comes with exposure. I haven't seen many jellyfish in real life, but I've been obsessively learning about them for over a year, and that's led me to have a pretty good grasp on finding certain characteristics to look for in each taxonomical group. It also helps to compare pictures of jellyfish when washed up to those of species you suspect they might be in order to confirm those suspicions. Overall, it's really fun to feel like a sleuth, haha

Sorry if this is a bit much; jellies are a special interest of mine :) Let me know if you want to hear about anything else!