This was a series of books where the reader travels through time, usually to the past (I know of one where the reader was sent to the future) to solve some mystery.
In one the goal was to learn the fate of an escaped slave from the American South. The disappeared slave was a male, named Thomas something.
In another the reader was sent to find the wreck of a Spanish galleon carrying a fortune in silver.
There was only one "successful" ending per story, if the reader was not careful he might end up trapped in a loop from one period to another unable to make any progress- what the series referred to as "stranded in time".