With an abundance of colors or only a limited number, granny squares are always spectacular. Stephanie Göhr, Melanie Sturm and Barbara Wilder made a wonderful new book about it: Häkelideen mit Granny Squares.
I just got the Dutch translation. In English the book is called Granny Squares: Over 25 Creative Ways to Crochet the Classic Pattern. It gives patterns for several types of squares and a great variety of projects. For beginners even the stitches are explained.
From bags to a cushion cover, from a beanie to a wrap and from pouches for glasses or a phone to bowls and a mug cozy. I particularly love the slippers and how they're constructed.
Once you get the hang of it, you can go on forever. Be careful not to go overboard.