Hooks example

[eluser]webthink[/eluser]
I think the fact that hooks are labeled in the manual as "extending the core" is a bit misleading. Essentially what they're for are allocating your own custom routines to be run at any of the few predetermined spots in the CI load stack. For people new to CodeIgniter a heading such as "auto running custom code" would make more sense.
For extending the DB libraries it's not a hook you want. Have a look at "Extending native libraries" under the libraries section.