... Qhr gb gur jvgpure'f npgvbaf jr unir ab nygreangvir ohg nfxvat gur Cebsrffbe'f tebhc naq bhe Ynql Sevraq sbe nffvfgnapr, nf jryy nf sbe nvq va fcrrqvat hc bhe erfrnepurf ba gur Cebwrpg. Nf sbe gur znggre jr cerivbhfyl qvfphffrq — jr ner tvivat lbh gur pbbeqvangrf bs bhe frpgvba va Zrepunagf' Dhnegre. Jr jbhyq yvxr gb erzvaq lbh gb hfr gur zntvpny obbxyrg. Guvf vasbezngvba vf gbc frperg! Jr vafvfg lbh uheel.
There is no visible signature on the letter.
It is actually encrypted with ROT13, here's decrypted text:
... Due to the witcher's actions we have no alternative but asking the Professor's group and our Lady Friend for assistance, as well as for aid in speeding up our researches on the Project. As for the matter we previously discussed — we are giving you the coordinates of our section in Merchants' Quarter. We would like to remind you to use the magical booklet. This information is top secret! We insist you hurry.