I think you'll find that the kind best to have is the kind most suited to you personally.
Mine is very religious and very interfaith. I have very few rituals or spells, and I do not have correspondences - I view these as helpful tools but not as being sacred themselves. My BoS has prayers, a few very key rituals like the Great Rite (both Wiccan and Thelemic), an overview of holidays, and key principles from several religions (Lord's prayer, the Shahada, the Gnostic Creed, etc).
I have a digital copy of my BoS. (I'll have a hard copy when I leave the nest, but for now it's a bit risky to have one...fundamentalist Christian family-while I am a Christian, I'm certainly not a Bible-thumper! Jesus wouldn't like Bible-thumping at all!) It's very much a work in progress; it contains my dedication, rules concerning my magickal practice, day-to-day entries (which include the moon phase, sunrise, sunset, daily Scripture application, daily readings and more), spells I made, spells I borrowed, short essays about various aspects of magick, references to books I'm studying in this path, poems and correspondences-I'm constantly adding to it! So my BoS is very personal and real at the same time.