The Book equipment item says it grants a +5 to Knowledge checks relative to the books specific (non-fiction) content.

How do I reflect that on the skills section of my character sheet?

So far it shows only a +2.  Is there a way to link my book to the skills calculation so that the sheet will display a +7 instead?

The best way to do that is to add a custom skill. There's a little gear icon at the bottom of the skill section. Call it book of lore or whatever. Then add the releative skill associated and the relevant modifier for the book. 

Thank you.