Field Guide meaning

The format of a field guide and its entries is likely to be what they're getting at, as well as it being small enough to be taken with you, but more in-depth an a pocket guide.

With little or no general material (that mostly consisting of entries about common features of related species) they contain an entry per species. Each entry is of the same format, typically consisting of one or more illustrations, some data such as size, when they're present, etc., and a description of the creature. The description is more free-form but still set out in a fairly predictable way. The idea is that once you're familiar with a guide, parsing an unfamiliar entry is quick.

Such a guide should be complete, for some value of complete (it may cover all common species in a given region, for example).

I have seen "field guide" used as a metaphor in relation to books about other things, though I struggle to see how it would apply in this case. At the very least each entry should start with some basic data and an example, before a couple of paragraphs of more descriptive text.