solution: a closure, that cuts of all surplus parameters

get_partialRight_right_04.js

_.mixin(
    { 
    "preserveCallersFirstNParams" : function (func, n) {
        n = n || 0;
        return function () {
            return func.apply(this, Array.prototype.slice.call(arguments, 0, n));
        };
    }
);

var getNumberOfPages = _.preserveCallersFirstNParams(
    _.partialRight(
        _.result,
        "numberOfPages"
    ),
    1
);

_.map(books, getNumberOfPages);
//32, 36, 32