function joinArrays(a, b, keyA, keyB) {
  var mapA = _.keyBy(a, keyA);
  var mapB = _.keyBy(b, keyB);
  var mapsMerged = _.merge(mapA, mapB);
  return _.values(mapsMerged);
}

var dbBookInfos = [
  {
    "isbn": "978-1982137274",
    "title": "7 Habits of Highly Effective People",
    "available": true
  },
  {
    "isbn": "978-0812981605",
    "title": "The Power of Habit",
    "available": false
  }
];

var openLibBookInfos = [
  {
    "isbn_13": "978-0812981605",
    "title": "7 Habits of Highly Effective People",
    "subtitle": "Powerful Lessons in Personal Change",
    "number_of_pages": 432,
  },
  {
    "isbn_13": "978-1982137274",
    "title": "The Power of Habit",
    "subtitle": "Why We Do What We Do in Life and Business",  
  }
]; 

joinArrays(dbBookInfos, openLibBookInfos, "isbn", "isbn_13");