The focus is on human biology: primate and cat systems feature, as the best-studied experimental subjects, but there's only the occasional comparative glance at invertebrate visual systems and no attempt to chart the evolution of the vertebrate eye. The approach is also "bottom-up", from neurobiology, anatomy and physiology, without clinical or cognitive perspectives. This focus, however, allows McIlwain to go into surprising depth for a short book, and to treat some "advanced" topics; and each chapter has a further reading list for those after more.
An Introduction to the Biology of Vision is a textbook, and quite a densely packed one. It's a readable volume, however, with clear explanations and effective diagrams: anyone with a basic understanding of the nervous system who wants to delve into the workings of human vision should find it a useful guide.