As with the other two 'Save the Cat' books, this is about mainstream, commercial screenplay structure. Although that definition can be extended to include many art house movies, these are not books about the art house. Readers looking for radical departures from the filmmaking establishment should search elsewhere.
The first 'Save the Cat' set out to show the common features in how a lot of successful film are constructed and it did so in an entertaining and clear cut way. 'Save the Cat Goes to the Movies' presented a wider range of film examples in each of Blake's own genre categories.
What 'Save the Cat Strikes Back' does is expand on those sections in the first two books (especially Book 1) that some people felt needed more depth. It has more detail in the way different parts of a story interconnect, how they can mirror each other and reinforce ideas. As with the other books the emphasis here is on structure; the ideas are yours, this is a guide to arrangement and presentation.
As this is Blake's last book, due to his early death in 2009, there is also a poignancy to some examples and anecdotes dealing with life and, in a few cases, death. Blake's central point being that stories are about life and its lessons, including its limits.
Leaving that aside though, I think this is an excellent expansion on the original book and well worth reading if you liked 'Save the Cat'. Of course if you didn't, then you probably aren't reading this review. :)