they are probably hidden in some academic publications, but see Microsoft Reseacher Stephen Robertson's list of publications at
http://research.microsoft.com/users/robertson/
he developed some ranking algorithm for Microsoft
here are some additional worthy read:
An Algorithm for Full Text Indexing
http://citeseer.nj.nec.com/529554.html
Charming Python: Developing a full-text indexer in Python
http://www-106.ibm.com/developerworks/xml/library/l-pyind.html