Some of the resources we've annotated are very hard to find, especially ones that are decades old.   Many books published before 1970 do not have ISBNs.   Many other resources that were published years ago may not have standard identifying numbers either (like a DOI or an ISSN).   If a resource is redily available, we've tried to provide you with the information you'll need to find it.   If you are looking for an older book that is difficult to find, we recommend starting with BookFinder.com. BookFinder is a very useful online tool that we use frequently.   If you are looking for old articles, pamphlets, papers or similar, try a Google search or check out the Mary Baker Eddy Library.