Non-stoichometric compounds
There are several resources available for this, for example, searching in Google Scholar yields over 37,000 articles (this is without any filters for age or specifics).

Organisations such as the American Chemical Society have a searchable publications link; related, CAS has a searchable reference list and ChemWorx which is a way to manage your research.  A similar set up is available with the Royal Society of Chemistry.

Individual universities also have searches/tutorials - for example, The University of California, Davis have a 'ChemWiki'.  Additionally, university libraries also are often accessible to the public and have search functions for journal articles, books.  Some universities, such as the University of Connecticut have Research Database Locator.

As mentioned in the comments, Wikipedia, particularly if a page is written well, have several links to journal articles and books etc.