Not a bound collection of testimonials (9,4)
I believe the answer is:
reference book
'collection of testimonials' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for reference book that I've seen before include "Authoritative non-fictional tome for consultation" , "A source of facts" , "Encyclopedia, say" , "Where to find information" , "Volume providing information" .)