When guitarists see the word 'riffs' they probably think of short phrases rather than complete pieces of music. '12 bar riffs' might bring to mind the thought of classic runs and phrases used by blues guitarists. This book isn't really looking at that sort of 'riff'. Instead, it consists of a set of 12-bar PATTERNS rather than short extracts. Having said that - and despite the accompanying CD performances being rather clinical and lacklustre, the examples are useful and very well graded from beginner-level through to intermediate. Think of them as 'skeletons' upon which you can build and into which you can inject some life and energy, and you won't go far wrong.
In my opinion, this book does have definite merit, but its title may mislead you if what you really want is a collection of classic phrases or extracts from classic blues songs and performances.
If what you really want are some classic 'intros' 'turnarounds' and 'outros' in the styles of, say, Muddy Waters, Eric Clapton, the Rolling Stones etc. you might prefer to hunt down Jimmy Hendrick's 'Essential 12 bar blues' (Wise publications).