Chapter 6 explicitly addresses SOA and modularity. It builds on the idea of "architecture all the way down" discussed in Chapter 3 (available for download below). In fact, if you look at Figure 3.2 in the excerpt, you'll see where modularity fits within the context of SOA. Modules tend to be finer-grained than services. Modules are a unit of intraprocess reuse. Services are coarser-grained than modules and are a unit of interprocess reuse. A service is composed of several modules.
In general, I find that teams spend quite a bit of time designing service interfaces, but don't spend enough time design service implementations. The result, as I like to say, is that within each service awaits a rotting design. Modularity increases the quality of the service implementation.