You thought correctly, @Aspirety! ^^
The book included in the first edition release of CLANNAD (the ‘CLANNAD Official Guidebook’) is a fanbook which provides a variety of information on the development of the visual novel and its characters. It contains some artwork and concept sketches from Itaru Hinoue, though many of the images focus on individual character expressions. Information on each of the main characters is provided including impressions from the scenario writers and limited information on their routes.
The single coolest aspect of the book is the lengthy production log co-authored by Jun Maeda and Kai. It covers the many ups and downs of the production process and provides unique insight into the creators’ lives during that period. Well worth a read. The origins of the proposed character ‘Clannad Man’ are also described in this section, so that in itself is worth looking into!
There is some content overlap between the CLANNAD Visual Fan Book and the Official Guidebook, but not too much. There is a stronger focus on art in the former and more of a descriptive focus on the latter.
Let me know if there’s anything specific you wanted to know about the book!