How do I update a node (book or Page) before saving it without touching the module code in Drupal? -

Before actually writing data to DB, I would like to set some value in the node. I am already working to modify book_nodeapi but I want to do it out of the code, some option options which are untouched to leave the module code, would be great.

Thanks

If I understand what you want, then in such works Module can be very useful. All you need to do is make a condition according to the type of content you want, and to determine the value that you do for such a task. Setting the value can be done using static values, values, or even php code.

There is a screencast with the rule basics.