The Silkie (1969) by A. E. van Vogt

Silkies. A race of apparent humans with the ability to change into other forms. One can live underwater. Another can survive and travel unprotected in outer space. In all three forms the Silkies can wield mental powers over energy to some degree.

Humanity has assimilated them by means of the Special People, who can establish telepathic rapport with the Silkies. All Silkies are male, and most are married to women of the Special People. They are employed as police in space, and most are comfortable with that role. One dedicated Silkie, Nat Cemp, encounters three different alien races, and with each encounter he gains more powers and learns more about the true nature of the Silkies, and of the Universe.