peripheral; only slightly connected to the subject; digressing
I feel that chapter six of your book is tangential to your argument, and should be cut from the manuscript and published as an independent paper.
tangere (to touch) (Latin)
Word Tangent (ial) taken from maths. A tangent is a line that touches a circle with a 90`degree angle. So the meaning comes from there only slightly connected, not central, preipheral.