This is Dover's reproduction of the authoritative Breitkopf & Härtel editions of these pieces. The above reviewer is right--the Brahms pieces are set much larger than the Mendelssohn. But there are some nice advantages to this edition.
-In the Brahms chorale preludes, the chorale text is written directly over the cantus firmus.
-There is a critical commentary, which also includes translations of the chorale texts.
-Mendelssohn's instructions for registration are reprinted before the Six Sonatas. These instructions are extremely useful for German romantic music in general.
The Schumann pieces included in this volume were originally written, not for organ, but for pedal piano-forte--an instrument that Schumann loved, but which failed to gain popularity.
Note also: Although this volume contains all of Brahms's organ works, it does not contain all the organ works of Mendelssohn and Schumann. In particular, it does not contain Schumann's beautiful canonic etudes (highly recommended).