Amazing book. He's a frikkin' genius writer, because he's so uninhibited and confident and smart that he gives you a brain-dump of everything in his mind. Take from it the bits you like, run with 'em, and have fun making some awesome shots.
If you haven't used a lot of flash before, you'll sometimes read over a page and have no idea what he's talking about. WTF? Not that he's overly technical... kind of the opposite, that he's so gushing and enthusiastic and dropping all the hip terms for everything ("start with a bit of a hot rim and then back it off, 'cause in a sidelight situation it's gonna blow it out by a stop... then it gets piped backed to the lens and baby, it's dark out there!"), that it's hard to bring him out of orbit and back into the land of 'OK, what button do I push?' But stick with it. Read the book, shoot, read it again, shoot some more. You'll get it.
McNally gets a lot of attention for using flashes in extraordinarily complex setups -- and yeah, he does. But he's always focusing on the people... the story... the eyes. He's not a landscape photographer. His stories about interacting with his subjects (models, celebrities, musicians, quarterbacks, astronomers, bagpipe makers) are what this is really about.
This book has longer stories, more details and more diagrams compared with the previous books. If you don't have his other books (Hot Shoe Diaries, or The Moment it Clicks) and you want to learn his techniques, *get this one instead*. It's fatter, it's got more writing, and the narratives are longer and more intricate. This one is more chapter-based with various techniques, and the other two are closer to "here's a cool photo, and here's a page about how I took it." If you have the other two and love him, then get this one since it's essentially all-new material, and his technique and philosophy are so useful and inspiring, that the more you read and see of his work, the better your photos will end up as a result.
*** Important note: McNally uses only Nikon and makes only passing mention of Canon. Everything is virtually interchangeable, *but* there's one important difference about flash exposure you need to know if you're a Canon shooter. All over the book, he's talking about the EV exposure compensation being a global adjustment (e.g., p. 213, 345) -- that is, if you change the EV on the camera, you program underexposure into the flash as well. That's how it works on Nikon, but *not* on Canon!
On Nikon: the camera EV and flash EV are indeed linked: lowering the camera EV lowers the flash output. So, to highlight the foreground, you go -2 EV on the camera, and then back up +2 EV on the flash to compensate.
But on Canon, this is *not true*: the camera EV and flash EV are independent. Dropping the camera EV drops the ambient exposure, but keeps the flash output the same! So to do the same as above on Canon, you want to do -2 EV on the camera, and leave the flash at 0 EV. If you do what McNally says, you'll end up over-flashing your subject on Canon.
This difference is *not* well documented, but you can find some more info on it at Canon's web page -- Google for "Canon EOS speedlite system tips" and click on the tips by photographer Stephen Wilkes, and there are a lot of sample photos for how this works. Neither system is better or worse -- but you do need to be aware of the differences!
*** Update February 2012. Nikon's new D4 will ship soon. The D4 offers the option to set the flash level using the Canon way, not the Nikon way... that is, on the D4, doing a -EV on the exposure will now leave the flash EV unaffected. Nice change, since it means you need to do one adjustment, not two, to lower the ambient level. It appears that this is an option (not a full-time change), and that this applies to the D4 only, not the D800. For details, search for an article called "Exposure Compensation When Using i-TTL Gets Easier with the D4" on Nikon's site.