Principles and Practice of Endocrinology and Metabolism
This is my second favourite endocrinology textbook and it is a great complement to Werner and Ingbar's The Thyroid. It covers the entire endocrine system in health and disease and is somewhat broader than The Thyroid. This book is considered to be one of the foremost books in the field.
It is has been useful to me on numerous occasions and sits on my bookshelf along with a few other endocrinology textbooks.