Westerburg is now in the German state of Rhineland-Palatinate. During the time of the German Empire it was in the Prussian province of Hesse-Nassau and Family Search has indexed its records under that jurisdiction. Until 1803 it was an independent sovereign territory (Grafschaft).
For a list of helpful publications about Westerburg, see Geschichtswerkstatt-Westerburg: Literatur
See also the pamphlets in the very helpful series Westerburger Hefte
The following three transcripts can be found at the Ev. Kirchengemeinde Westerburg office in Westerburg along with other church record books.
“Kirchenbuch der ev. Gemeinde Westerburg 1641-1699.” Transcript. 9 August 2000. Ev. Kirchengemeinde Westerburg office, Westerburg, Germany
“Kirchenbuch der ev. Gemeinde Westerburg 1700-1752.” Transcript. 9 August 2001. Ev. Kirchengemeinde Westerburg office, Westerburg, Germany
“Kirchenbuch der ev. Gemeinde Westerburg 1753-1809.” Transcript. May 2002. Ev. Kirchengemeinde Westerburg office, Westerburg, Germany
The transcripts above are just that typed transcripts of the original church books and are much easier to read than the originals.
See Kirchenbuch, 1641-1876 for the microfilm copies of the original church record books. The early records are written in the old German script and the entries are in a mixture of German and a little Latin.