Birds
Hawaii’s Birds: Considered the definitive field guide to both native and introduced birds, by the Hawaii Audubon Society. Also be sure to check out their in-depth books on both the Kolea and Manu-o-ku.
Field Guide to Birds of Hawaii: by the American Birding Association.
Hawaii Birds (Pocket Naturalist Guide): lightweight and laminated.
Migratory Shorebirds: small printable by Ariel Illustrates.
Checklist: Garden Birds: free printable checklist by Ho’omaluhia Botanical Gardens.
Don’t forget about the eBird and MerlinID apps!
Native Plants
Flowers of Hawai’i: A durable, easy-to-use field booklet created by Laulima.
Amy Greenwell Garden Ethnobotanical Guide to Native Hawaiian Plants & Polynesian-Introduced Plants: Clear photos and descriptions of native plants and their uses.
All Flora
I found these two books by Mark Merlin to be very helpful in identifying common plants in Hawai’i, both native and introduced:
Hawaiian Coastal Plants
Hawaiian Forest Plants
Mushrooms of Hawai’i: An Identification Guide
From Bess Press:
Hawai‘i Wetland Field Guide
A Field Guide to Hawaii’s Coastal Organisms: Algae and Invertebrates
Guides/manuals recommended by others (I haven’t seen them so can’t vouch for them)
Hawaiian Plant Life: Vegetation and Flora
Manual of the Flowering Plants of Hawai’i: Two-volume set, very comprehensive reference manual.
Other
First Aid and Medical Treatment of Injuries in Hawaii: Books about first aid by the wonderful Susan Scott and her husband Craig Thomas, MD. Bonus: on her website is an archive of her long-running Ocean Watch column from the Star Advertiser.