I've played every Nintendo Pokemon game, and I have guides for most of them. The Prima Official Game Guide for Pokemon FireRed and LeafGreen is the worst guide I've seen.
The guide will get you past all the gym leaders and tell you where to catch Articuno, Moltres, and Zapados, but that is as far as it goes. It says on the cover "maps for every area," but I've found at least seven areas that it does not have maps for. Next, on the cover it says, "hard-to-catch Pokemon found," but I have so far found wild Pokemon in areas that the guide says you cannot find them. For example, in Viridian City, it says you can catch Gyrados, Horsea, Magikarp, Psyduck, Slowpoke, and Staryu, but I have caught Slowbros in that water as well.
I have also found several glitches in the guide. For example, it says you can catch Gyrados in Fuchsia City, but the picture of Gyrados is really a Weedle. The maps show where some of the items are hidden but I have found items in places not shown on the map. Another annoying thing is that it does not list the level of the legendary birds, so I don't know whether to try and capture them now or later.
In the Pokedex section of the guide, it says that some Pokemon can be caught on Seven Island and Six Island. What does that mean? The guide does not say anything about a Six or Seven Island. All they tell you is that some Pokemon can be caught there. Does that mean it's a hidden Island? Does that mean you can catch some Pokemon, but the guide writers don't know where? No map or description is included in the guide.
The maps don't always show where all the passages and paths lead. The maps do not show where some caves go. At the edge of some of the maps it says "to town," but it does not name the town.
The guide only tells you the name and location of the items, but not what it does. I have found items that are not listed in the guide. It tells you different events, for example if you talk to somebody you will get a Pokemon, but so far I have found at least three events that it does not list. It tells you the name and level of every trainer's Pokemon but it does not say the attacks or items of each Pokemon. Every other guide I've seen at least has the attacks of the elite four and your rival. But this guide only says the name and level; it does not say the attacks or items of any Pokemon. It does not even say what level the Pokemon are that you get in events. Overall, don't get it. I am planning to get the official Nintendo guide; when I sell the Prima guide, be sure not to buy it. I have used the Pokemon Ruby/Sapphire Official Nintendo guide and it is everything I need.