I've been reading Osha Gray Davidon's "Under Fire: NRA & The Battle for Gun Control." It's a 1993 work, and thus doesn't cover the field since then -- pretty much ends with the Brady Act.
The author is strongly antigun, but except in a few places, doesn't let it affect his writing, that I can see. (He even acknowledges that "cop killer bullets" were an invented issue, and NRA was right on that). He does have some incredibly good sources for his material. I could only find one minor error in his history -- he has two firings occurring on the same day, when in fact they were a couple of weeks apart.