I enjoy nice field guides almost as much as going out into the field and really admire the accomplishments of the authors in combining art and utility. Many of the guides already described above are among my favorites, but two of my favorites not mentioned yet are:
San Diego County Native Plants by James Lightner - locally comprehensive and compact with a knack for highlighting key distinguishing characteristics between species. Those are rare features among plant field guides in the Western USA - Newcombe’s guide manages this in the Northeastern USA.
Tracks & Sign of Insects and Other Invertebrates by Charley Eisenman and Noah Charney - this one unlocks the IDs on a whole segment of the natural world that was previously a mystery to me (weird things growing on plants, etc), and is particularly useful as it is organized by features and not taxonomic group. I’m hoping for a similar treatment of fungal/microbial plant pathogens.