A Spellbook, Book Of Shadows, or whatever you call it, should contain information about your magick discipline. Deities, colors, oils, tools etc.
Of course, you should also write the spells you made/making. Along with your experiences. I, myself is a Satanist, so I contain doctrines of Satan in my book.
You should also contain the articles you might need to re-read in your Book.
Hope I helped.