I've never bought a GTA guide before, but in the past I've never wanted to complete a GTA game 100% (that is, find the hidden packages, do all the stunt jumps etc), but I've never needed to, but with the achievements on the 360, this guide is essential for getting 100%.
Play.com is the cheapest place I have seen the guide for, and it was an impulse buy which I bought whilst I was buying something else, but it was worth it. The guide is extremely informative and tells you about every single thing you can do in the game, from random characters, friendships, weapon/health/armour locations to multiplayer tips. The guide has it all.
However, all the above could be found out with a quick Google search, but I think it's better to have the guide - it's colourful, easily laid out, easy to follow and it saves time visiting ten different websites until you find what you're looking for (and nine times out of ten it'll have been robbed from the guide anyway).