The "wild side" of pet guinea pigs refers to the fact that the book discusses wild cavies - several species - and how pet guinea pigs are related to them, as well as being a manual for the young pet owner.
This is a pretty good book - couple of pieces of incorrect info, but nothing harmfully wrong. There may be some confusion for American readers because it's a British book; some of the pictures show a style of outdoor hutch that's common in England but not in the US, and a few other such things.
The book includes lots of technical terminology, and includes the stuff many books avoid, such as the necessary feces-eating, and guinea pigs' place in the wild food chain. There's an excellent glossary in the back, giving further definitions for most of the technical terms in the book - in fact, the glossary part could serve pretty well as a reference for an elementary school child writing a school paper, and there's a drawing of a guinea pig skeleton that will also be of interest to a science-minded child. The best part, though, is the photos - lots of domesticated GPs, but also lots of the various wild cavy relatives, from Patagonian cavies to capybaras.
There are also trivia sidebars - did you know that Queen Elizabeth the First had a pet guinea pig? All around, pretty interesting, more so than most books aimed at younger pet owners.
It's part of a series that also includes the "wild side" of pet lizards, ferrets, cats, dogs, and fish.