The LDS Family History Library has begun digitizing and making available on
the Internet all of the Family History books in their collection. About 5000
books have been digitized and are available, and they have announced that
they are adding about 100 titles a week to the on-line Family History
collection.
http://www.lib.byu.edu/fhc/
If you don't find what you want there, you may find Family Histories
online helpful. Go to
http://www.rootsweb.com/~ote/family_histories/
A Full-Text search for "colquhoun" got 414 hits, "calhoun", 4245,
"calhoon",
97, "cahoon" 843.