It was either written by a man who came to be known as
Thomas à Kempis, or Kempis compiled and edited the writings of some late 14th-Century
Dutch priests. The version posted here is one of many different English translations (from the "original" Latin -- Dutch was probably its original language) over the centuries.
This is a public-domain etext, available in HTML, PDF, RTF, plain text, Windows Help, and Hypercard formats, digitized by Harry Plantinga (hplantin@calvin.edu -- at least at the time), 1994, and available at the Christian Classics Ethereal Library, whose URL is http://ccel.wheaton.edu/
(This will get posted over the next few weeks, with hard links added here and there, from time to time; there will be a lot of dead links for the time being.)
Book One
Book Two
Book Three
Book Four