A Books of Shadows is a "religious text and instructions for magic rituals found in Wicca." Since it's inception, somewhere around 1949... the definition of a Book of Shadows has changed in the public context.
With a myriad of published Books of Shadows starting in the sixties, ("The first attempt at a mass produced BoS appeared in pamphlet form in 1964.") to the scary, (albeit fabulous) movies of the seventies (one of my favorites is "The Curse of the Crimson Altar"), to shows like "Charmed"... a Book of Shadows today extends far beyond that of Gardner's Wicca. Any type of witch can have one, and usually do, if not several. There are online versions, download and print versions, beautiful handwritten versions, post-it note versions (hahaha) and so many more.
And then there are the Grimoires. There's even a more modern book called "A Grimoire of Shadows" by Ed Fitch. Hell, yeah!... just mash those two together. (It's a pretty good book, btw.)
The distinction is sometimes made that a grimoire just lists magic, spells, herbs, rituals and ALL the things you keep in your book of Shadows.... BUT.... your Book of Shadows also contains personal reflections, and notes, and thoughts, and feelings, and results of your magic.
I don't make the distinction. I will say "grimoire" if I'm feeling story-teller-y and history-ish, and I will type the heck out of BOS all day long. Technically, every magic book I have, even the printed ones I've purchased, would be considered a BOS since I have notes in almost every margin.
I've already written in my Book of Shadows without doing this... can I do it now?
You can consecrate anything you have at any time... just start with a cleansing first. After the first one, you can opt for gentler cleansers (like sage). I love consecrating (blessing) my magical tools. I do it often. The more you do, the more you use them, the more you handle them, the more you spiritually connect with your magical items, the stronger the bond you develop.
I don't have a blank page at the very beginning to add a pentagram.
Here's where my motto kicks in.... Improvise: Use the tiny little open space on your very first page to draw a small pentagram or use the cover itself or make a permanent cover. Adapt: Tape, sew, glue in a front page. Overcome: If you really want to do this, pick one of the previous suggestions or start anew.
My High Priestess use to make the most beautiful books of shadows. She included protection herbs within it's cover. Consider incorporating protection herbs in your books; either in your handmade cover or in a bookmark.
Here are a couple of links for other Book of Shadows related consecrations and blessings because seriously... mine is straight out of "Mastering Witchcraft."
Purification & Blessing of your Grimoire
Book of Shadows Blessing
Mastering Your Grimoire: My Grimoire's Book Blessing