This is a hard culture to find accessible source material on.   Three of these works are now out of print.  The Swanton and Hammond works have both been out of print for over 90 years and are considered rare collectable books.  In libraries that have these two works you need special permission to see them.  The Ricklis book is recent and quite good and up to date and out of print now.   The Newcombe book was written in 1958 and has out of date information and some unintended racist bias.  It was good in 1958, but so many more primary sources, in Spanish archives for example, have become available since then and a tremendous amount of archeology has taken place since then.  All this has changed what we know about the Karankawa.  

Hammond, Charles A. , "The Carancahua Tribe of Indians" The Karankawa Indians, Archeological and Ethnological papers of the Peabody Museum. vol 1, No. 2 , Cambridge Mass, 1891, Ed. Gatschet, Albert S.

Newcomb, W. W, "The Indians of Texas", University of Texas Press.

Swanton, John R. " The Indian Tribes of North America", Smithsonian Institution Press, Washington.

Ricklis, Robert A. "The Karankawa Indians of Texas", University of Texas Press, Austin.