Sharp book vs soft book
A sharp book posts low-margin, quickly-corrected prices used as a reference; a soft book has higher margins and slower lines that lag the market.
Pinnacle and betting exchanges are the classic sharp references. Recreational US books move later and carry more vig. Arbitrage between books mostly appears when a soft book has not yet moved to where the sharp market already is — which is why the same few book pairs account for most of the opportunities ArbMonster records.
Measured on ArbMonster: Which books pair up most
Related terms: Vig · Steam · Closing line value