I seized the quiet time during Christmas holidays to do some reading. One item that was on my reading list for quite some time now is Marty Cagan's 2005 white paper on the role of the product manager called Behind Every Great Product. In this paper Marty first wrote down tenets of product development that he developed at Netscape and later refined at Ebay and which eventually resulted in his book Inspired: How To Create Products Customers Love.
In the white paper Marty describes roles and responsibilities of the product manager, and characteristics of good product managers. So if you're looking for a description of the product manager's role that is more extensive than Horowitz's Good Product Manager/Bad Product Manager but less voluminous than the book Inspired, the Behind Every Great Product white paper makes a great read!