PL:Have you read Jess In Action by Earnest J. Freedman-Hill? It's one of the better books out there on rule engines, though it does require some experience to get the most out of the book.
A variation on the bookstore could be recommendation engine. That would provide an opportunity to use a rule engine for direct matching on category/subcategory and aggregations. One potential way of aggregating recommendations would be to calculate the weight of the top categories based on the last 100 purchases. The weight can then be used to filter recommendations. A simple recommendation could just show the top 10, but to get more accurate recommendations, it would be good to look at the subcategories and perform additional filtering. For example, I buy a lot of kung-fu movies, but I don't care for Steven Segal. If Amazon were to recommend one of his movies, I'd probably end up ignoring all the recommendations. Instead I could write the following rules.
if the user has written reviews of movies in the category, get the aggregate weights of all the user's categories.
if the top ten movies in the category have a positive weight, include it in the recommendations.
if the movies purchased by the user is greater than 5% for any of the actors in the film, increment the item's sort priority.
if the lead actor in the movie is rated negatively by the user, exclude the item from the recommendations.
JAM：I also buy a lot of kungfu flicks. Maybe a rules engine could help settle a never ending dispute on which are the best movies of all time. Would seed it with the following movies:
Five Deadly Venoms
Master Killer
Kid with the Golden Arm
72 Desperate Rebels
Last Hero in China
Iron Monkey
Once upon a time in China - Part Two
18 Bronzemen
I would like to see something aimed at the health care industry. Rules galore
PL：I don't need a rule engine for that. The movie with the best fighting has got to be.
Drunken Master II (aka, The legend of the drunken master)
If you like Last Hero in China, I'd recommend Fong Sai Yuk with Jet Li. I consider it Jet's best movie. I've always wanted to try to make a program to invent a kung-fu movies (as a joke). I'd throw in a list of fighting styles, some actors and then the rule engine could spit out a script :)
Joking aside, if you do decide to write a book on business rules, perhaps I can help. For the last year, I've been considering writing a book on rule engines and rule technology. I was going to cover the low down nuts and bolts of a rule engine and how different types of rule engines perform. This way, it should show when a procedural rule is appropriate and when Rete is needed.
Actually, the health care industry already uses Rete rule engines. They just don't say it. The insurance industry like wise has been using Rete rule engines for over a decade starting with Art's rule engine. CLIPS was inspired by Art's engine, which later inspired JESS.


Comments：PL is an excellent guy I have met.
                                   -  Nicky