A few good starting points for studying real-world dragons:
Tales of Great Dragons (J. K. Anderson) - Yes, it's a coloring book, but a very informative one, covering dragon tales from around the world. As starting points go, you could do much worse... plus, dragons to color!
Dragons (Peter Hogarth with Val Clery) - An older (1979) "coffee table" book, but it has a decent overview of dragons, with some good pictures.
Dragons - Truth, Myth and Legend (David Passes) - Geared toward kids, this covers the basics of dragon lore and legends.
Dragons: A Natural History (Dr. Karl Shuker) - Many dragons from around the world are covered here, often with pictures.
If you really want to delve into the origins and symbolism of dragon myths, you might want to dig deeper. Joseph (sometimes Joe) Nigg's books collect information from classical sources, following the "evolution" of the dragon through different cultures and ages. And there modern bestiary-type books, like
Here Be Dragons: A Fantastic Bestiary (Ariane and Christian Delacampagne), though they usually include other animals, not just dragons and kin. You can even dig deeper and try to track down original source material from whatever culture you're focusing on, but that might lead down a rabbit hole (or wyrm hole) of no return...
Recent kids/YA books like the
Dragonology series (by "Dr. Ernest Drake," a.k.a Dugald Steer) draw on dragon lore, but modernize and sometimes water them down for modern audiences. Joe Nigg wrote one as "John Topsell", entitled
How to Raise and Keep a Dragon, which isn't too bad and covers a broader range of cultures, but is still watered down enough I wouldn't rely on it exclusively. And the there are sources like D&D manuals and such... good for inspiration, but not if you're after authentic dragon lore.
Good luck!
ETA: Tracked down a decent web site (many of my old favorites are defunct):
The Circle of the Dragon
