Just a suggestion, but how about 'recommendation letter' ?

It is not a letter, and not necessarily a recommendation as it is also written when you receive bad marks. It is just a formally necessary document for doing the marking ...
something the examiner or supervisor has to write to explain his judgement.

http://en.wikipedia.org/wiki/Letter_of_recomm...
A recommendation letter or letter of recommendation is a letter in which the writer assesses the qualities, characteristics, and capabilities of the person being recommended in terms of that individual’s ability to perform a particular task or function. 

http://en.wikipedia.org/wiki/Dissertation#Germany
The required submission for the doctorate is called a Dissertation or Doktorarbeit. The submission for the Habilitation is called Habilitationsschrift.

For me,"letter of recommendation" sounds fine and I don't really see a difference between a "Habilitationsschrift" and the "recommendation letter".

But if it's supposed to be called "Habilitationsarbeit"
here are some other suggestions:

Siehe Wörterbuch: Habilitationsschrift

Siehe auch: Habilitationsschrift

Sorry, coppied the wrong term from wikipedia

- "Habilitationsarbeit" --> "Habilitationsschrift"

I don't know if this is what you mean, but this is officially what I got with my academic transcript from my university when I applied to do a Masters. It's written by a tutor/lecturer and is basically a statement regarding your ongoing academic progress.

Hallo,
ich würde die Diskussion gerne weiterführen, da die Antworten noch nicht ganz das zu treffen scheinen, was wirklich mit einem Dissertationsgutachten gemeint ist.
Es ist ein Dokument, dass die Dissertationsschrift kritisch beurteilt und somit die Notengebung begründet. Am Ende des Gutachtens wird die Note auch aufgeführt.
"Reference" scheint mir nicht ganz passend, denn das Dokument beurteilt nicht den Doktoranden.
Könnte man für "Gutachten zu der Dissertation von (NAME)" vielleicht schreiben "Opinion on the dissertation by (NAME)." oder gibt es angebrachtere Übersetzungen?