Don't underestimate field guides! I find that they help me learn and retain information much better than apps that aim to do the same. Plus, they're beautiful… and you get to show off to your friends when they seem them in your bookshelf — or grabbed from your backpack on the trail! Why mostly illustrated rather than photographed? Usually illustrations can better showcase colors, forms, and behaviors. Plus, again, pretty! Links from Bookshop.org, which supports independent bookstores.
Sections
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General North American Field Guides
California Field Guides
Outside North America Field Guides
~Almost~ Field Guides
General North American Field Guides
I love my Peterson Mammals Field Guide! Did you know Peterson is basically the father of the modern field guide? Must have for anywhere in the US.
Peterson Field Guide To Mammals Of North America
For the whole of the continental US and Canada, this is your go-to bird guide. A pretty hefty book, this is a household must-have.
The Sibley Guide to Birds
Honestly, I love love love my NatGeo field guide (1st edition — but 2nd edition is great, too!). Is it the most popular, no, that would be Sibley's, but it gets the job done with illustrations I sometimes even like more than Sibley's (shh!). Note that this is only for the western US and Canada.
National Geographic Field Guide to Birds of the WEST
Do you live in the western US or Canada? You need this field guide! Why not just get one for your state? Birds don't recognize borders — this will be much more helpful in the long-run.
The Sibley Field Guide to Birds of WESTERN North America
Likewise, if you're in the eastern US or Canada, this has got to have a place on your shelf or in your backpack.
The Sibley Field Guide to Birds of EASTERN North America
This is a chunky book, but I love it! Categorizes by family and is quite comprehensive. Super useful!
Trees of WESTERN North America
Likewise, this (even bigger) guide is perfect for getting to know trees — backyard and “in nature" — in the eastern US and Canada. Very helpful in my dendrology course!
Trees of EASTERN North America
The iconic Peterson strikes again with amphibians and reptiles. For western North America.
Peterson Field Guide To WESTERN Reptiles & Amphibians
The iconic Peterson strikes again with amphibians and reptiles. For eastern North America.
Peterson Field Guide To Reptiles And Amphibians EASTERN & CENTRAL North America
A great starter book for people of any age getting interested in nature. Just make sure you/they know that many of the species in this book can be invasive in natural areas, so admire them but don't plant them! :) Woman author.
Fylling's Illustrated Guide to Nature in Your Neighborhood
California Field Guides
Though this book uses photographs rather than illustrations, I have found it to be quite useful and well-organized.
California Plants: A Guide to Our Iconic Flora by Matt Ritter
Off to the Sierras? This is your book!
The Laws Field Guide to the Sierra Nevada
Beautifully illustrated book that will also be helpful in the field. And, new!
Butterflies of the Bay Area and (Slightly) Beyond by Liam O'Brien
These field guide pamphlets can be hit or miss, but I think this one does a great job highlighting common species and illustrating them well. Easy to bring into the field, a great starter wildflower field guide!
Common Wildflowers of California Brochure by Lisa Acree
Great beginner book for tidepooling in California! Woman author.
Fylling's Illustrated Guide to Pacific Coast Tide Pools
This waterproof inverts guide is key for tidepooling!
Mac's Field Guides: California Coastal Invertebrates
Outside North America Field Guides
It's a little chunky to throw in a suitcase, but I love this field guide and it covers the whole continent!
Birds of Europe by Lars Svensson
Alas, this book uses photographs rather than illustrations, but I've still found it to be quite useful and portable in Hawai'i!
A Photographic Guide to the Birds of Hawai'i by Jim Denny
This was quite helpful in the Galapagos!
Wildlife of the Galápagos
~Almost~ Field Guides
Beautifully illustrated classic California “field atlas” worth having in every California house.
The California Field Atlas by Obi Kaufmann
I just received this for my birthday, and I already love it! The best Nature Study Guild book I've seen!
How to Read the Wilderness by the Nature Study Guild
Iconic California illustrator. Not exactly a field guide, but still an effective, creative way to learn more about nature.