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Over the last 30 days, the US Patent Office has published several patents for Amazon that lend insight on the company's future plans for its Kindle book reader.

One patent, filed in December 2006, will grant customers that purchase a print copy of a book the ability to access the electronic version as well.

Two other patents go into detail about incorporating targeted advertising in on-demand content on Kindle.

Under the terms of the patents, Amazon could inserts ads throughout its ebooks: in the margins, between chapters or every 10 pages, from beginning to end, for example, notes MediaPost.

An additional cross-referencing feature would avail users to supplemental material, annotations, illustrations and the ability to print paper copies on-demand in PDF or other formats.

Such ads may benefit users in unforeseen ways. According to Bill Slawski of SEO by the Sea, a novel that details a restaurant could additionally be equipped with food or dining ads, for that restaurant or others. Users could also be incentivized to view ads in their books with a lower access rate.

Ad tracking would rely on bar codes or some similar tracking model, right on the ads. Amazon's patents also include details on how users could interact with the offerings to receive more information.

Amazon has released its fourth quarter earnings, which reflected an earnings jump of 42 percent — to $5.67 billion — from the same time last year. Profits doubled to $207 million.

For the entire year, revenues grew by 39 percent to $14.84 billion, marking Amazon's fastest annual growth rate since 2000.

Earlier this year Amazon reported a profitable Q4, in great part because of the Kindle, which according to Jeff Bezos is "outpacing our expectations." Kindle launched in late 2007 and experienced so much popularity in its first year that the online retailer had trouble meeting demand.