NOTE: Only the first 9 answers are significant.  The 10th most
frequent pair occurs once and therefore there are *lots* of
possibilities.

1.  CD JPLITIHV9jBBtvOQp6dM     BOOK z5686ar5QZvGSf4Rug3v       6
2.  CD LzqnS7admb6M9K3Ih0tB     BOOK eT5VH4ICf9w3F7S3Z31b       5
3.  CD 4Y4sFWqlQVU4nK4RpPF5     BOOK 1aBkUXD3UzPLRiC13gL0       5
4.  CD sb12T6iNFOBrx1RluEnY     BOOK UdT9ZtsEgaWIvlcOLssX       4
5.  CD 1NFxFjXZhaBfJit8cl19     BOOK B5mDxIieYaW2VzXIuIZt       4
6.  CD xXpgSx6Lo560Ciq57nVI     BOOK cVWpdiyQeWNInVX5S8l9       2
7.  CD NXl3KmouImUVef07FfWW     BOOK 6nDKFHkUggGr5TvdxhVB       2
8.  CD SpILqpUkJqeJBOMlpKcs     BOOK MHkCpLCHupjogjtuDNMM       2
9.  CD KST0mJGRWpNqRXqZBPcV     BOOK CKT6oMRBk8XdHyqqKxF9       2