Now that DONUTS the book is out (yay!) I thought I'd provide some links to a few of pieces of equipment and specialist ingredients I recommend you get if you're going to try making the recipes.
Baking Tray for baked cake donuts.
Donut cutter for cutting out yeasted donuts (our original recipe).
Mini donut cutter for cutting out yeasted donuts (our original recipe).
Digital thermometer for determining temperature of oil when frying yeasted donuts.
Dough scraper for slicing through yeasted dough and cleaning up dough-y covered surfaces.
Savarin ring tin for the giant vanilla donut cake.
Chunky sprinkles for birthday cake donuts
Oil based food colour for chocolate
Disclaimer: This isn't a sponsored post - everything that's recommended here is because I personally think it's brill and we use it ourselves in the bakery.