Pokemon Sun and Moon is to receive another guide which will outline the entire Alolan PokeDex including movesets, abilities and post-game information.
The first guide covered most of the stuff you needed including where to find Zygarde cells but it did forget to show exactly where the Ultra Beast locations where and is really needed. If you want to full rundown of the new Pokemon games you could buy the first guide and then the second with the new Ultra Beast information but you can probably get away with the first one and some online Google action.
It is not sure why they did not include the beasts in the first guide but bringing out a entire second guide for the same price as the first one seems a little excessive. If you need some starter advice, check out our Pokemon Sun and Moon tips guide for useful information.