Tiling: In my experience with tilers you have to be VERY careful that they are doing what you want or you will end up with something that is literally set in stone and not what you want. First of all, research carefully your tiles and buy everything you want in ONE batch. Under European guidelines there can be up to 1cm difference in tile width or length which will mess up your plan if you add a slightly different sized tile to your extension.
Always watch for edging. Some tilers will recommend plastic “baguettes” but there are other options that work equally well.
Colour of your grout: choose carefully and make it clear IF you don’t want grey – the automatic default.
Once the grout has been laid INSIST that it is covered with plastic so no dirt gets on it.
Patterns. I have not found a book on tiling designs. Some good sources for ideas have been the LaPeyre and Hornbach catalogues (of all places) as they dare to show different tile patterns that can work better than plain old horizontal or diagonal.
Ironically I find the tile catalogues the least imaginative or too over the top.