Please, before attempting to upgrade your whole site to Drupal 6, read these notes and every comment in this thread carefully.
The database layout for outline and book modules have dramatically changed between Drupal 5 and Drupal 6. As a result, the upgrade involves moving a lot of data around. A full upgrade path for outline module is provided between the two versions.
Note: One specific configuration of D5 outline is not supported during the upgrade because of a limitation of D6 book module. In D5 outline, it was possible to add several pages at the very top level of the outline. Usually, one single page would serve as the topmost level, and all other pages in the same outline would be added as children or grandchildren thereof. However, it was possible to have several nodes at the topmost level. In D6, the very core logic that powers book.module does not make it possible to have this feature, i.e. by design, only one node can be at the topmost level of a book outline. As a result, it is not possible to import D5 outlines where there were several nodes at the top levels of some outlines. It is strongly recommended that you resolve this issue before you upgrade the site to Drupal 6 by reorganizing your outlines and checking that for each outline, there is only one node at the top level.
Because of all the above, it is imperative that you test upgrading your site with a copy of your database first!
1- backup your database.
2- make a copy of your site somewhere.
3- test the upgrade path to Drupal 6.
4- make sure everything works as expected.
5- If you experience problems with the outline upgrade, check this very issue.
6- Repeatedly test the whole upgrade until everything works smoothly.
7- backup your database again.
8- do the upgrade on your live site according to usual guidelines.