Andre Norton 
The Time Traders 

To anyone who glanced casually inside the detention room the young man sitting there did not seem very formidable. In height he might have been a little above average, but not enough to make him noticeable. His brown hair was cropped conservatively; his unlined boy's face was not one to be remembered—unless one was observant enough to note those light-gray eyes and catch a chilling, measuring expression showing now and then for an instant in their depths. Neatly and inconspicuously dressed, in this last quarter of the twentieth century his like was to be found on any street of the city ten floors below—to all outward appearances. But that other person under the protective coloring so assiduously cultivated could touch heights of encased and controlled fury which Murdock himself did not understand and was only just learning to use as a weapon against a world he had always found hostile.