I have a copy of this book. Would you like information on something specific?
You can sometimes find a copy. There is a store in Downieville called the Ruffled Goose. You might try and find a telephone number for them (the area code is 530) and see if they have one. They may be willing to ship it to you. I saw one of Sinnott's books in this store (I believe it was this book), and I believe the price was $65.00. They are out of print and are very hard to find. You might also try on e-bay or on one of the used bookfinder sites. They usually run $70-100+.
The author was James J. Sinnott. A member of a pioneer Sierra County family, he was a life-long Downieville resident. An educator and an accomplished author and historian, he authored several excellent books (six?) about Sierra County.
The Mountain Messenger is still in publication and has an office in Downieville. They have many of the old papers in the office in hard copy (but they are not in very good condition) and have quite a few on microfilm (in their office as well). I have visited their office several times and they have always been very helpful. They have a VERY old microfilm reader that is usable but is not in very good shape.
I believe the California State Library also has microfilms of this newspaper. It has been publshed since the 1850s.