My son, age 9, is naturally fascinated with dragons. Every month, when we go to the local bookstore, he chooses two books on the subject. His favorite dragon books are those by Dugald Steer: Dr. Ernest Drake's Dragonology (Candlewick Press, ISBN 0-7636-2329-6) and Dragonology Handbook (Candlewick Press, ISBN 0-7636-2814-X). Two more books are due to come out in this series this month: Obscure Spells and Charms of Dragon Origin (Andrews McMeel Publishing, ISBN 0-7407-5731-8) and Dragonology Tracking and Taming Dragons Vol. 1 (Candlewick Press, ISBN 0-7636-3233-3).
What's so special about this series of dragon books? For my son, it's the use of runes and deciphering coded messages. He also loves how the author treats dragons as though they are real in the flesh. The Dragonology book contains "samples" of dragon skin, wing webbing, and dragon's dust. There are secret compartments containing letters and mini-books inside the book. My son spends hours at a time going through these books.
Gail Gibbons' picture book, Behold... The Dragons (Morrow Junior Books, ISBN 0-688-15527-8), is perfect for children who are too young to read Steer's Dragonology books. In this book, children learn the basic history of dragons and their place in mythology and folklore. Brightly illustrated and clearly written, it's the perfect read-aloud book to help foster a child's interest in dragons.
For teens, there are a wide range of dragon fantasy books on the market. However, you may also want to check out Dancing with Dragons by D. J. Conway (Llewellyn Pub., ISBN 1-56718-165-1). In this book, Conway teaches the reader about the history dragons and the different types of dragons found in mythology and folklore.
Finally, for adults who are interested in doing further research into the history of dragons, there's an excellent etext available of Ernest Ingersoll's book Dragons and Dragon Lore(1928). Enjoy!