I can't help but would say Google the authors' names to see if they really are involved in the breed. Some breed books are written by people who's never even kept the breed yet are described as experts by the publishers.
Best book without a doubt would be by Margaret Norman. This lady who died not long ago had the Francehill kennel of Shelties which was one of the top winning sheltie kennels. Her last big winner was the Blue Merle Francehill Two Tone who after her death went to live with Rosalie Brady and is still shown by her. HTH.