I used the dimensions of a random paperback book so that I can mount my final piece once it’s printed. The book I chose has the following dimensions:
height 19.6 cm
width 12.8 cm
spine 1.5 cm
I then opened a new document on illustrator and used the dimensions accordingly, because I was designing the front and back I added times the width by two and added the width of the spine. The height stayed the same. After getting the document set up I added a few ruler guides to help separate the front, back and the spine. I also added bleeds manually to help me keep most of the information centered and not too close to the edges I gave the bleeds a 1 cm width on both sides of the front and back covers.