Unless you play FFX while online, I would suggest the Brady Guide, but seeing you are from Ireland, wouldnt you have the PAL version with Dark Aeons? If thats the case, I know there was another english guide, but I cannot recollect the name.

As for the switching part, just use the characters you want to use. In my playthrough, I only use 4 main characters, Yuna, Tidus, Wakka, and Rikku. I level Kimahri up for his grid only, just for his solo battle. I was never into using Lulu much, and sometimes, instead of a Wakka playthrough, I use Auron (for a second Gen FFX).