Last year, I found an 1859 Post Office Directory at the Making of America website sponsored by the University of Michigan. Making of America has an incredible collection of scanned and searchable books. In the 1859 directory, I found the Cash Creek post office listed. This fascinated me because the histories I've read about Cash Creek indicate the mining camp started in 1860.
Well, I used the 1859 date in my proposal for the Pikes Peak Regional History Symposium. I thought I'd better check it again and kept looking in Google Books for the directory and it didn't show up. This morning I remembered it might have been in Making of America and, sure enough, there was the directory with Cash Creek listed. Looking closer at the entry I saw "Cash Creek, Lake, Colo." This time, the Lake County and Colorado caught my eye. Colorado was a part of Kansas Territory in 1859. I went to the title page and found that the Post Office Directory was published in 1870. Ugh.
I'm planning a trip to the Rocky Mountain Regional Office of the National Archives. They have the microfilm of the reports of site locations for post offices. There is no guarantee that the papers for the Cash Creek post office are included (some early post offices aren't), but it is the next step for my research.
Research Tip of the Day: When searching on-line books, always look at the title page for source information and PRINT it!