This work was not published complete (i.e. in full score and incorporating all of Mahler's later revisions that didn't make it into the original Kahnt score) until 2000. So for Mahler's final thoughts on the work one would have to shell out for two volumes totalling more than $300. But at least the work IS still in print, unlike, say Brown's important critical edition full score of Haydn's Creation (from Oxford, which seems to be a music-publishing black hole -- just try to buy a copy of Vaughan-Williams' Sinfonia Antartica).
You can see a rather substantial preview of the Weber/Maher here (including all of the substantial introduction to the edition as well as much of the libretto and its translation and many score pages from Act I):
http://books.google.com/books?id=AGoXUs ... ry_r&cad=0
The second volume contains the excerpts of the rest of the music as well as the critical report and can be previewed here:
http://books.google.com/books?id=UN-XCn ... ry_r&cad=0
To judge from a quick glance at the critical report, the original 1888 Kahnt score (which WOULD be in the PD if one could track it down) has many problems, not limited to wrong notes. And, more important, it doesn't incorporate Mahler's revisions, which were in a list he sent to Universal Edition.
--Sixtus
PS: While we're discussing early Mahler, the IMSLP version of Mahler's Das klagende Lied is of the two-movement version. The original score of the 3-movement version incorporating the later suppressed initial Waldmarchen scene would be in the public domain if one could track it down (my copy of the 3-movement edition is from the non-PD Mahler Critical Edition). And the later-removed Blumine movemen tof the 1st Symphony I believe was first published in the 1950s or 1960s, making it non-PD in many areas.