It contains the rough outline of a sector-spanning campaign (really rough -- the whole thing is only about 4 pp long), a detailed history of the Fifth Frontier War including tactical maps of fleet positions at various points in the war, a description of a naval squadron (including some broken HG ship designs), a description of the Al-Morai merchant line, and some reprinted material -- the 4518th Lift Infantry Regiment article from JTAS 9, the map and UWPs of the Spinward Marches sector from Supp 3 (but there are also bits of info here and there that weren't in Supp 3), and the civilian career charts from Supp 4. I think that's it. 48pp total.
This book is something of a hodgepodge -- you're almost certain to find at least something useful and interesting in it, but you're just as certain to end up ignoring significant chunks. I'd say if you're a fan of canon and the OTU and the Spinward Marches that this book is worth picking up if you get a chance, but I wouldn't pay too much for it on eBay. There's nothing absolutely can't-live-without essential, and it's going to be reprinted by FFE eventually anyway.
One last noteworthy bit: if you're using GT
Behind the Claw as your main source of Spinward Marches canon, be warned that that book is incompatible with SMC in several places, some small some big, so if you don't like canon conflicts you might want to just save yourself the headache and not look at SMC at all (since it's not really a conflict if you don't know the conflict is there
).