Making the top-level book page index organized by categories (taxonomy), instead of just weight

I'm upgrading a Drupal website for a client from 4.5 to 6.x, and could not perform the upgrade, so am recreating it in a clean Drupal 6.x install (the site is fairly simply structured). I've almost completely duplicated it, except for one thing: Their Drupal is like an online magazine, with different issues, each of these a top-level book page. I can get /that/ structure, but what they have done, is somehow organized the book pages by taxonomy categories in the index (see http://www.majorensis.com/numero1).

I asked them how they did it, but they could not contact the person who originally made the site, so I'm on my own. The original Drupal install is very fragile, and barely works, and I have not been able to see how they did it originally. I do know, however, that they did not use any additional modules.

So my question to you guys is: any ideas on how I could implement this? The categories are not actually part of the book structure, they are only present on the index (therefore, the categories are not actual pages of the book). I've thought about using Views, but there has got to be an easier way. And besides, they did not use Views/Nodelist to create the by-category index. Any ideas?

Thanks,
Daniil.