Joseph V. Homan, Paul J. Kovacs – A Comparison Of The Relational Database Model And The Associative Database Model (6 page article). Issues in Information Systems, Volume X, No. 1, 2009. pdf 
> This paper compares the relational database model with the associative database model. This paper briefly summarizes the relational and other familiar data models. The remainder of this paper will introduce and describe the associative database model. The associative model is less known because it is relatively new and does not have a large software supplier base. While it seems to offer a number of benefits and advantages over other database structures, it has yet to become a commercial success in the mainstream database market.
Simon Williams – The Associative Model Of Data (24 page article). Journal of Database Marketing, Volume 8, 4, 2001
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> Lazy Software has created the Associative Model of Data, the first entirely new database architecture since the advent of the Internet. Its product Sentences is a multi-user, web-enabled database management system written in Java, and is the first commercial implementation of the Associative Model.
Using Sentences, customers can design and develop sophisticated database applications more quickly and with less technical know-how than has previously been possible. The high cost of application development has forced many companies to adopt packaged solutions that are not well tailored to their needs. Sentences brings custom applications back within their economic reach.
Simon Williams – The Associative Model Of Data (book, 284 pages). Lazy Software, 2nd edition, 2002 pdf