Contribute to endless books on Scotland's earliest serfs (6)
I believe the answer is:
helots
'serfs' is the definition.
('helot' can be a synonym of 'serf')
'contribute to endless books on scotland's earliest' is the wordplay.
I cannot really see how this works, but
'books' could be 'ot' (Old Testament) and 'ot' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for helots that I've seen before include "They worked like slaves" , "Ancient Spartans, in hotels?" , "Slave labour" , "Serfs" .)