Find the word definition

Wiktionary
field guide

n. A book intended to help the reader to identify wildlife or other natural objects.

WordNet
field guide

n. a guidebook describing natural objects of some type that might be encountered in the field; "a field guide to mushrooms"

Wikipedia
Field guide

A field guide is a book designed to help the reader identify wildlife (plants or animals) or other objects of natural occurrence (e.g. minerals). It is generally designed to be brought into the 'field' or local area where such objects exist to help distinguish between similar objects. Field guides are often designed to help users distinguish animals and plants that may be similar in appearance but are not necessarily closely related.

It will typically include a description of the objects covered, together with paintings or photographs and an index. More serious and scientific field identification books, including those intended for students, will probably include identification keys to assist with identification, but the publicly accessible field guide is more often a browsable picture guide organized by family, colour, shape, location or other descriptors.