Transcript follows. Incidentally, Ebert's write-up of said visit can be read here.
To read more letters by Bukowski, a superb book with which to begin is Screams from the Balcony.

Transcript
BARFLY
Cannon Publicity Department
A LETTER FROM A FAN
I must say that in the production process of BARFLY, I had some fine luck. First off, the director, Barbet Shroeder, insisted that my contract contain a clause that no part of BARFLY, as I had written it, could be changed without my permission. This was adhered to faithfully. When I was not on the set, I was contacted via telephone for my o.k. on the smallest of changes, most often just a word here and there in the dialogue. I agreed to most minor changes and when I did not, things were left as I wished them. If this was not a heaven for the writer, then there will never be one.
The other part of my luck was the actor who played Henry Chinaski. Mickey Rourke stayed with the dialogue to the word and the sound intended. What surprised me was that he added another dimension to the character, in spirit. Mickey appeared to really love his role, and yet without exaggeration he added his own flavor, his zest, his madness, his gamble to Henry Chinaski without destroying the intent or the meaning of the character. To add spirit to spirit can be dangerous but not in the hands of a damned good actor. Without distorting, he added, and I was very pleased with the love and understanding he lent to the role of the BARFLY.
Also, the whole cast and crew seemed to feel so good, so up, during all the shooting. It could be felt at all times. More than a few onlookers spoke to me about this crazy high that seemed to exist. Roger Ebert came by one Friday night during a shoot. He stayed many hours. We watched the shooting, talked between takes. He said to me, "I've never had such a good time." It was in the air, it was everywhere and the delight and magic of it was caught, I felt, in and on the film itself.
BARFLY, I love you.
Charles Bukowski
(Signed)