There's a magazine called "Guns And Bullets" which increases your Small Guns skill. You can find or buy them throughout the game. What I want to know is if it is worth reading the magazine until I reach 91% or higher before I start spending skill points on my Small Guns skill or if it is better to start investing in the skill as soon as possible.

In Fallout 1 not really. Skill points are cheap so just invest them in the skill. Also there's no skill point inflation when skill is higher than 100% (unlike Fallout 2) which again results in you using fewer skill points. In Fallout 2 I'd say 90% of the time also no, the exception being some strange build where you don't have any weapon skills tagged and skill-wise you're spread too thin so it might be nice to save some skill points by reading the magazine. That's a very meta build, for your usual small guns-speech-lockpick build there's no point in delaying investment in small guns.

That's really helpful advice. The last few playthroughs I tried depending on skill books, but it really hampered my combat skill in the beginning.