Is the book called "Brewing Classic Styles?" If so, those recipes are designed for a 6 Gal full boil and, if I had the capacity, I would boil 6 Gal. The author (Jamil) is accounting on the volume lost in the boil as well as throughout fermentation.
I have used those recipes for partial boils (I don't have capacity for full boils).
I boiled 3.5 then topped off to 5.5 Gal (while checking OG to make sure I hit the target).
By the end of fermentation (omitting trub, volume lost to dry hopping, in the racking process, etc) I had just over 5 Gal going into bottles.
Hope this helps.
Amarillo Pale Ale
A Stout I brewed for a guy as a lesson
Jamil's Belgian Tripel
Centennial Pale Ale
Imperial Red Ale