General Resources

• Our guide to gall identification
• Detailed descriptions for our key filters
• Glossary for plant and insect terms
• Our reference library
• Advice on finding galls
• Documenting Trees (Oak focused) for identification

Books

• If you are new to galls Charley Eiseman and Noah Charney's Tracks & Signs of Insects & Other Invertebrates: A Guide to North American Species is a good place to start learning. It covers a lot more than just galls and is an excellent resource.
• Russo's guide to galls of the Western US

Non North American Resources

• To ID galls and other plant symptoms in Europe, visit bladmineerders.nl