Published volumes of the Ruhi Institute books, in PDF. (Links to documents, offsite.)
These books have appeared on five websites (as of 2013). Some of these sites are problematic in that they have pre-publication copies of the books or are annotated by individuals hostile to the Faith, so only two links are shared below.
To learn about the Ruhi Institute and how to use these materials, see the official website ruhi.org. See more Ruhi materials at ruhiresources.org.
Ruhi book series
Columbia: Ruhi Foundation, 1987-2012