Zoology 1 covers the classification system and binomial nomenclature, which are used throughout the 3 zoology texts. It also introduces terms sucah as "extinct" and "habitat." The subsequent texts assume that the student knows these things.  You can download and read Lesson 1 of Zoology 1 to learn these concepts (except for extinction), before moving on to Zoology 2 and Zoology 3. Then it really doesn't matteer which order you read the books. This Lesson is available at the bottom of the main page at www.Apologia.com listed under Resources.