OK, it looks like I was a little hasty! I had read the other posts about Lenny and had tried 0.6.17 but with the same result. Then I saw Garywitscher'spost and tried 0.6.29 and it runs.
So, anybody else trying to run Calibre under Debian Lenny amd64 should download this file
and extract it into the directory /opt/calibre. Running the shell script 'calibre' should start the program.