As Zimmerman wasn't arrested, what was he "booked" for?
Technically, he was arrested/under custody.
I just watched the video, and looked at a still of him standing with his back to the camera. It would be difficult to determine there were no bruises, but you can see there isn't a laceration (at least not a significant one). He may have had a small laceration or abrasion; bleeding from a scalp injury can be pretty impressive, even from small wounds.
I don't believe the police report lied that there was blood on the back of his head, it would have been washed off by the paramedics when they evaluated him at the scene. The news report also says police said paramedics did not recommend that Zimmerman go to the hospital. In any case, he didn't appear as someone who'd just had the snot beat out of them.