What is allosteric regulation?

This question really boils down to semantics, and the definition can be clarified by discussing enzyme regulation in general. The 3 main ways that enzymes can be inhibited are through the following mechanisms: competitive inhibition, non-competitive inhibition, and uncompetitive inhibition. In competitive inhibition, the inhibitor binds directly to the active site and blocks the substrate from binding (so they are "competing" for the active site, hence "competitive inhibition"). Non-competitive and uncompetitive both involve the inhibitor binding to a separate regulatory site on the enzyme that is different from the active site (the second sentence of your book's definition of allosteric regulation). However, we have to differentiate between the two, and a nice, concise delineation can be found here. This page states:

While uncompetitive inhibition requires that an enzyme-substrate complex must be formed, non-competitive inhibition can occur with or without the substrate present.

Therefore, your book's definition is more in-line with non-competitive inhibition. I'd encourage you to look at this page for a more thorough read, and because they have a nice simple illustration that might help put it in perspective.

Here's a more abbreviated answer:

Inhibitors may change the active site to prevent substrate binding, but that's not the only mechanism of inhibition. Specifically, non-competitive inhibitors may limit the ability of the enzyme to carry out its action without changing the shape of the substrate binding site (see this figure).

Final thought If you actually look at the etymology of the word "allosteric," its word roots come from the Greek allos that means "other or another" and from the Greek stereos that just means "solid or object." So in actual etymological terms, it's a more general term that should apply to both uncompetitive and non-competitive inhibitors because both bind at some other or allosteric site on the enzyme. However, the way your book defines it is more like the definition of a non-competitive inhibitor.