There's lots of good info in that book. I use mine regularly, but should get a new one. You just can't use those values as though they are the gospel. They're just a starting place. For example, I had a gun that listed in the Blue Book as being worth $350. I traded it for a complete AR lower + $400. Sometimes people just want things.
In the end value is defined by only two people; the guy with the stuff and the guy with the currency. I can't tell you the number of times I've disagreed with the book number. Lots of times guys have told me, "But the Blue Book says it's worth $." I always respond with, "OK, but I'm asking $$$. You won't hurt my feelings if you don't buy it." About 80% of the time they meet my price because that's what it's worth to them.