SPSG is responsive to the problem that many people in the software industry are thrust into positions in which they are given responsibility for the outcome of a software project but are not given any formal or informal training in how to make that happen. SPSG provides an introduction to the steps that successful software projects follow that can be read by both technical and nontechnical readers.
Unlike Code Complete and Rapid Development, SPSG does not provide you with limitless options. It lays out one specific approach to software development that works pretty well for most projects most of the time. The plan described in SPSG is designed to address the most common weaknesses that software projects face. The beginning project leader can use the approach described in SPSG as a pre-fab project plan. The extremely knowledgeable software developer will sometimes be able to come up with a more customized development plan than the one described in SPSG. But the plan described in SPSG will work much better than no plan at all, and "no plan at all" is the most common alternative.
If you read Rapid Development ... the first 100 pages of SPSG will be about half review--presented differently, of course. The remainder of the book is mostly new content that is organized to be used as the basis for a project plan. Please be sure to study the contents and the detailed table of contents to be sure the book contains enough new material to be worth reading.