This is a series of (primarily) system-agnostic material detailing topics in Gloranthan history, culture and myth.
Each volume is derived from Greg Stafford's notes, and includes substantive amounts of previously unpublished setting background.
These volumes have a quite complex publication history and multiple "editions" or "versions" or "printings" of each volume are known. There has not been an authoritative correlation of existing versions, etc. The ca. 2015 PDF "reprints" are primarily re-creations from source materials of early printings, sometimes with corrections (notably, The Stafford Library Volume 02: The Glorious ReAscent of Yelm "did get a number of Greg's handwritten corrections incorporated into it, but none of the others have had a similar treatment").
N.B. The covers of each volume have changed with various reprints. In some cases, they are entirely different from the original covers; in most cases, they are notably visually distinct.
Rick Meints is the editor for the 2015 reprints, and has stated "I have been asked how many different editions/versions/printings exist for these, but the answer is a rather disappointing 'we don’t know', and we probably never will. They got printed in very small batches over a very long period of time. They were printed by different photocopy shops using different paper, binding, cardstock, etc."