You need to have the book in your hands, or the magic doesn't seem to work. Keeping it in your possession is just as vital as a warrior's axe, or an archer's bow. Scrolls can work in a pinch, but the cheap things seem to fail even as the spell manifests.
Look closer. Run your fingers gently over a page. You're not imagining it, you're definitely feeling something here beyond the weight of the arcane knowledge. You've cast this spell enough times you can practically feel it under your fingertips.
@Balina I like where this is going.
@Balina There are interesting ways you could go from it. A few guiding questions, should you want them: Did the people who wrote the book know how it worked, or were they just following an arcane formula they didn't understand? Is the book itself the source of the magic, or does it merely manipulate forces that already exist? What happens if the book gets damaged; is it capable of repairing itself, or is it possible to get it repaired by an ancient sect of warlocks or something?