This afternoon I was looking for some music that eventually led me to this collection published originally by Friedrich Pustet and re-issued by Noetzel Verlag in 1979. The collection is not one that organists are widely familiar with and I thought I should promote its virtues a little.
The collection contains 19 volumes of renaissance/early baroque organ music, which can be downloaded individually from the Petrucci Library website. Each volume contains repertoire proper to a certain religious order, court, geographic area or in some cases liturgical calendar. The typeset is friendly and its registration indications helpful.