This two-disc set is worth having at the price, especially as it seems Vols. 1 & 2 are not offered on Amazon. Really, Vol. 1 is the best of the five volume set. Vol. 2 is good also, but the sound is a little muffled. If you're really enthusiastic or curious about fin de siècle art music for the quintessential belle époque/art nouveau instrument--the harmonium d'art--then get this set, and look for Vol. 1 at an out of print music source.
Get the whole harmonium set: Works for Harmonium 1 Sigfrid Karg-Elert: Harmonium Works, Vol. 2 Sigfrid Karg-Elert: Works for Harmonium, Vol. 3 Sigfrid Karg-Elert: Works for Harmonium, Vol. 4 Karg-Elert: Harmonium Works Vol 5 Sigfried Karg-Elert: Compositions for Harmonium
The whole piano set: Sigfrid Karg-Elert: Piano Works, Vol. 1 Piano Works 2 Karg-Elert: Piano Works Vol. 3 Piano Sonatas 1 & 3