I'd been meaning to read this for quite a while, and when I saw that Audible Anarchism had an audiobook version, I figured I'd start that way. I ended up tearing through it (with the occasional backtracking to ensure decent absorption/comprehension) and it was really enjoyable.
If you've read Homage to Catalonia, this is an even more interesting read, as Rocker was commenting on the situation in Spain as it unfolded. Of course, it's a bit sad to encounter Rocker's high hopes for the CNT-FAI fending off Franco's forces in hindsight.
In any case, it's the strongest piece of theory in support of anarchism I've yet come across. Unlike works I've read penned by Bakunin, Goldman, Parsons, etc, this relies less on firey ideological rhetoric, and grounds itself instead in a history of anarcho-syndicalist movements and orgaizations while focusing on the tendency's particularities within the socialist traditions of the 19th and 20th centuries. It recalls Kropotkin in its push towards providing a path forward, but is obviously more recent than, say, Conquest of Bread.
Paperback, 116 pages
Published July 30, 2004 by AK Press.