I haven't read the book in detail, although I've read the course notes it's based on. He has you build (in Scheme) a number of interpreters for a variety of small programming languages. They all share the Scheme syntax, but they have different features than Scheme, so you can see the benefits of each specific language feature and how you can implement it. By the end, you've written many different interpreters, including a compiler for a functional language. (I'm not sure how this compares to SICP, but I get the impression that it covers similar ground.)
At the time, it was an eye-opening experience: I learned that a programming language is much more than just its syntax, and that adding features to a language has both benefits and drawbacks. The section on compiling programs functional programs by automatically rewriting them in the continuation-passing style was particularly enlightening.
Wow, so this is a little off topic, but I'm constantly rooting around. (Book looks awesome btw, downloaded, and 'hope' to read it).
But I saw that the book was 'free', and I wanted to take a beat on where free things were be published. I checked google with the quoted title, zero results.
Then I went to his site saw three links, download, download+donate, and buy paperback. I struggled here, he was giving me no excuse not to donate if I was going to read the book for free. Then I saw the paperback offer, and thought I don't really consider a "book" a 'book' unless I can put it on my shelf. Thus I would gladly pay for a paperback, and if I like his "book" that is how I would give my return. I want a memento from our "free" digital exchange too, I mean, if the book is worth it's salt.
But that's not the point, point is this guy published through 'LuLu', and LuLu like Google is a name that can mean something simply because it means nothing. Something I'll say to my voice command pc, or when I tell my friends fifty years in the "future" I bought a future-word-for-book off 'LuLu' (post computing ubiquity).
How are they going to win? Look at their community. When I got to the page and played around with the links, I was really blown away. I see this company making it.
This book is great. It's by no means written in plain english; It's definitely an academic text, but Shriram does a wonderful job of explaining how programming languages can be implemented.
(edit)
http://www.cs.brown.edu/~sk/Publications/Talks/SwineBeforePe...