Regarding antiques, long out of production firearms, and rare subvariants, the Gun Blue Book is still useful. It is also useful on production, rarity, etc. and how to train someone on condition. On new and used recent AR-15's, Glocks, Remington 700's, etc, and other mass produced firearms and frequent traded, not so much--gun broker auctions are probably more useful as well as weird limited production models such as Keltec bullpups and so on. One thing on GB is that you have to ignore most of the reserve listings as those often set around for a long time with little or no action unless particularly desirable. The Buy it Now with nosebleed prices are similar ones to ignore.
Regarding investment potential, old out of production firearms will appreciate, especially in firing condition, as they are still usable tools. The general rule is buy low and sell high. However, most people have exaggerated ideas about rates of return. You are lucky to get an average 3 percent real return after inflation per year. But using the rule of 72, you can expect doubling of your investment in 24 years. You also grab classics when they appear at low prices--e.g. the S&W service revolvers going for about $200-250 a few years ago or Mosins at 75-100 but you have to act immediately on out of production firearms. Another example is more recent JM Marlin marked lever actions especially some of the used pistol calibers and trapper models have also climbed in price due to the Remlin issue.
