Database
db={"author":[{_id:"asdasd2dqdfwefwe1",books:["ISBN2e242","ISBNasdasdasd"]},{_id:"asdasd2dqdfwefwe2",books:["ISBN2e242","ISBN1111sdasd"]}],"books":[{ISBN:"ISBN2e242",title:"some book1"},{ISBN:"ISBN2e299",title:"some book2"},{ISBN:"ISBN1111sdasd",title:"some book3"},{ISBN:"ISBN2e242",title:"some book4"},{ISBN:"ISBNasdasdasd",title:"some book5"},]}
Query
db.author.aggregate([{"$unwind":"$books"},{"$group":{"_id":null,"books":{"$addToSet":"$books"}}}])