Book collection

My girlfriend’s grandfather has sadly passed away, leaving behind a phenomenal collection of military history books, hundreds and hundreds,including what must be every conceivable book written on the Battle of Britain (he was ground crew on Hurricanes during the Battle of France/Battle of Britain). There are also signed Robert Taylor prints etc. What do people think is the best way for them to achieve a good return on such a collection? I don’t want to get too involved in their sensitive family business and start butting in with advice, but neither do I want some pillock to come along and give them peanuts for a lifetime’s collection. Are there specific collectors who would buy such a collection en masse?