I second John on this. It's mostly a reference book and a bibliographic gateway to coffee research; but I give it a big thumbs up.
Although the $85 price sounds steep, it's very well printed and very cheap by academic standards; my guess is either that Illy is subsidizing the print runs to some extent, or that it's selling a lot more copies than the average academic edition.
In the last year and a half, some HB regulars have been very focussed on grinders and the nitty-gritty of espresso extraction. In this effort, it turned out that the corresponding chapters by Petracci in the Illy book were key to understanding some of the more inexplicable things we were seeing. However, this wasn't information that popped out, or that you'd notice if you were reading the chapter just to find out how to make a good shot or what sort of grinder to buy. Instead, the book became very useful, at least to me, a step or two further into my coffee hobby, when my questions started to get more detailed. I think others will confirm this.
If you want a more accessible route into the latest in haute-espresso,
Scott Rao's book is the stand out choice. Scott has been a big part of the group trying to make everyday sense of the science, and his primer on espresso is far more grounded in on-going research than Schomer's ever was. I would regard it as the one must buy book for anyone starting out in espresso today.