One of my friends filled hers with any information she found pertaining to her religion, from spells to runes to quick notes she jotted down.
I know sometimes else who has a BoS filled with almost nothing but information on plants.
Mine (though I prefer to use the term grimoire, as people tend to assume that I'm Wiccan if I refer to it as a book of shadows) has three sections; one for spells, one for rituals (such as honoring the dead, celebrating the seasons, etc), and one for miscellaneous writings (I have a short rhyme about faeries in the misc section, as an example).
Some other things people put in their BoS is Zodiacal information, Moon phases, dreams, chakra information, and different types of stones (just some ideas; it's not limited to or required to be any of this).
Point being, there's no right way to go about creating a BoS. And you can always start over. I have an old, smaller grimoire that I decided was too messy and hard to use, so I made a new one. That's always an option.