[sbdmmg]

Thanks for your suggestion.
I tried both the ebuild too; unfortunately that doesn't get to the end of the installation, and for this reason I was trying the binary installation.

Indeed there seems to be an issue with the image magick libraries. I tried to work around this by setting LD_LIBRARY_PATH, but it looks like some of the libraries are still being picked up from the system. (I installed calibre in /usr/local/calibre/calibre/)

$ export LD_LIBRARY_PATH=/usr/local/calibre/calibre/lib:/usr/local/calibre/calibre/lib/ImageMagick/modules-Q16/coders/
$ LD_DEBUG=libs ebook-convert foo.html foo.epub

<...a lot of libraries being loaded from /usr/local/calibre...>

     21083:
        Looking for large trees in foo.html...
        No large trees found
Generating default cover
     21083:     find library=liblcms2.so.2 [0]; searching
     21083:      search path=/usr/local/calibre/calibre/lib:/usr/local/calibre/calibre/lib/ImageMagick/modules-Q16/coders               (LD_LIBRARY_PATH)
     21083:       trying file=/usr/local/calibre/calibre/lib/liblcms2.so.2
     21083:
     21083:     find library=libtiff.so.3 [0]; searching
     21083:      search path=/usr/local/calibre/calibre/lib:/usr/local/calibre/calibre/lib/ImageMagick/modules-Q16/coders               (LD_LIBRARY_PATH)
     21083:       trying file=/usr/local/calibre/calibre/lib/libtiff.so.3
     21083:       trying file=/usr/local/calibre/calibre/lib/ImageMagick/modules-Q16/coders/libtiff.so.3
     21083:      search cache=/etc/ld.so.cache
     21083:       trying file=/usr/lib/libtiff.so.3
     21083:
     21083:
     21083:     calling init: /usr/lib/libtiff.so.3
     21083:
     21083:
     21083:     calling init: /usr/local/calibre/calibre/lib/liblcms2.so.2
     21083:
     21083:
     21083:     calling init: /usr/lib/ImageMagick-6.6.5/modules-Q16/coders/png.so
     21083:
     21083:     /usr/lib/ImageMagick-6.6.5/modules-Q16/coders/png.so: error: symbol lookup error: undefined symbol: png_LTX_RegisterPNGImage (fatal)
     21083:     /usr/lib/ImageMagick-6.6.5/modules-Q16/coders/png.so: error: symbol lookup error: undefined symbol: png_LTX_UnregisterPNGImage (fatal)
ebook-convert: coders/png.c:2825: ReadPNGImage: Assertion `image_info->signature == 0xabacadabUL' failed.
/usr/bin/ebook-convert: line 9: 21083 Aborted                 $base/bin/ebook-convert "$@"
     21080:
     21080:     calling fini: /bin/sh [0]
     21080:
     21080:
     21080:     calling fini: /lib/libncurses.so.5 [0]
     21080:
     21080:
     21080:     calling fini: /lib/libdl.so.2 [0]
     21080:
     21080:
     21080:     calling fini: /lib/libc.so.6 [0]

Thanks for any suggestion you may have.