I looked for quite a while to find what I have here. I have actually three books that contain information on my Rosebrughs. The two mentioned previously, and a third, "The Scotch-Irish of Northampton County Pennsylvania," by the Northampton County Historical and Genealogical Society, 1926 (found just recently). All three are rare and difficult to find but it is possible.
First, use a good book-search engine like bookfinder.com (there are others also). Then, keep a want list of books and search for them on a regular basis. Sooner or later what you're looking for usually turns up in a bookstore somewhere. It's where I've found the bulk of my library. Surprisingly, even some of the rarest books turn up sooner or later.
Another idea would be to find a library that has a copy in need of repair or re-binding and offer to cover the cost in turn for getting a copy made at the same time (you pay for the copy you keep). It has to be out of copyright, but this shouldn't pose a problem for books of this age. Getting a copy made from one with a sound binding and in excellent condition will be harder to get someone to agree to.
Finally, check the Family History Library catalog at your local Family History Center to see if the book has been microfilmed. If so, order the microfilm. That way you can make photocopies of the pertinent pages. Hope this helps.