What thicknesses of leather are suitable for book binding?
Traditionally leather should be about 1.00 mm thick for bookbinding. I have used leathers that are much thicker as well as leathers that are thinner. The thinner leather poses its' own problems. It can tear quite easily. 

Years ago I did some bookbinding with some rather thick leather. Probably something comparable to what you have at the moment.

You should be able to use the leather at hand, but there are a few things one needs to be aware of first.

You need to learn how to tapper the edges of the leather with a sharp paring knife to finish the  turn-ins and corners. It takes some practice and patience. The paring knife must be sharp at all times.



The paring knife (third from the left) is similar to the one I used to use for tapering the leather. 

 

More Paring Knives!



Basic Paring Stages for Leather

Do the leather tapering (paring) in five stages and practice on some old pieces until you feel comfortable working with the knife.

PS. Be very careful.