Definition of Turing Machine as a language acceptor
Immediately after the definition, the text (fifth edition) continues


  This definition indicates that the input $w$ is written on the tape with blanks on either side. The reason for excluding blanks from the input now becomes clear: It assures us that all the input is restricted to a well-defined region of the tape, bracketed by blanks on the right and left. Without this convention, the machine could not limit the region in which it must look for the input; no matter how many blanks it saw, it could never be sure that there was not some nonblank input somewhere else on the tape.


This differs from the approach some other authors take, using a tape that's bounded on the left and unbounded in the other direction, and stipulating that the input string begins at the leftmost cell and continues to the first blank. I don't find Linz's definition to be better, if for no other reason than it eliminates any language containing the empty string from being the language of a TM.