This is the 545th most frequent Danish word.
selve
"Selve" means "the very" or "the actual" in English.
Here 'selve' emphasizes the subject as epitomizing the definition of kindness, adding intensity.
Han er selve definitionen af venlighed.
He is the very definition of kindness.
In this sentence, 'selve' underlines that it was the specific and exact reason.
Det var selve grunden til, at jeg blev hjemme.
That was the very reason I stayed home.
The usage of 'selve' here points to the book as being the actual object referred to.