Coahuila Archives and Libraries
One great resource for searching out your Coahuila ancestors is the General Archive of Coahuila (Archivo General de Coahuila). The website is available only in Spanish; click on "Documentos en Línea" if you want to see the original images of historical documents online.There are also online catalogs of the archive's collection; that way you can find out whether or not the archive has the documents you need without having to call or visit the archive.
Within Coahuila there are many different muncipial archives in the many cities within the state. You can view the directory of municipal archives, which gives the address, telephone number, and website of each archive.
If your ancestors are from Saltillo, the capital (and largest) city withing Coahuila, you can visit the website of the Saltillo municipal archive.