Hmmm, where to begin? Here are a few that I use a lot in the winter and/or that people might not be aware of (with a strong eastern/southeastern US bias, I’m afraid):
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Woody Plants of the Southeastern United States: A Winter Guide by Ron Lance. His illustrations are fantastic, and he is generous in defining woody plants + includes non-native/adventive species. Pro-level, I guess.
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Weeds and Wildflowers in Winter by Lauren Brown. Useful for those interested in dry fruiting stalks and other winter remains of herbaceous plants. Somewhat limited in scope but often gets me close, even if the specific species I have found isn’t covered. Novice-friendly.
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A Trailside Guide to Mosses and Liverworts of the Cherokee National Forest by Paul Davison, available online here. This is a great reference for getting started with mosses & liverworts in the southeast, with lots of good information and photos for each genus. Novice-friendly.
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Common Mosses of the Northeast and Appalachians by Karl McKnight et al. I always get more interested in bryophytes in the winter when there are not as many other things claiming my attention, and this is my go-to when I want more information than provided in the above. Semi-pro? (If you’re interested in mosses or would like to be, check out Robin Wall Kimmerer’s excellent book Gathering Moss - not a field guide, but a fascinating & accessible look into the world of bryology.)
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FloraQuest is an interactive, appified version of Alan Weakley’s Flora of the Southern and Mid-Atlantic States. It has a great interactive key with the option to geo-filter by region, and is connected to iNat so you can post an observation after keying something out. If you don’t know a term used in the key, you can tap on it to pull up a definition from the glossary. Limited to iOS, unfortunately, but would be worth getting a designated iOS device for if you’re an Android user (I don’t have a smartphone but bought an older used iPod Touch in part so that I could use FloraQuest). Probably most appealing to serious botanists.
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While I’m on electronic resources, I’ll mention NC State’s page of online keys & image galleries. These are mostly pretty focused on North Carolina, but some of the interactive keys that Dr. Krings has created are great, including excellent photos.
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Not a field guide, but I have to give a shout-out to Harris & Harris’s Plant Identification Terminology: An Illustrated Glossary. This is the best reference I have used for botanical terminology, and it’s relevant year-round. Well organized, with great illustrations. Where else are you going to find a “Key to Common Inflorescence Types”?