private string MyWebMethod(string input) {
    JavaScriptSerializer serializer = new JavaScriptSerializer();
    List<string> userNames = serializer.Deserialize<List<string>>(input);

    using(var db = new dbContext()) {
        var usersAndBooks = from u in db.Users
                            join ub in db.UsersBooks on u.UserId equals ub.UserId
                            join b in db.Books on ub.BookId equals b.BookId
                            select new Result { User = u, UsersBooks = ub, Book = b };

        var predicate = PredicateBuilder.False<Result>();

        // Construct predicate to filter usersAndBooks down to users we care about
        foreach(var name in userNames) {
            predicate = predicate.Or(r => r.User.UserName.Equals(name));
        }

        usersAndBooks = usersAndBooks.AsExpandable().Where(predicate);

        foreach(var name in userNames) {
            // Logic to construct final {User:{Books}} JSON object
        }
    }
}

var usersAndBooks = from u in db.Users
                    where userNames.Contains(u.UserName)
                    join ub in db.UsersBooks on u.UserId equals ub.UserId
                    join b in db.Books on ub.BookId equals b.BookId
                    select new Result { User = u, UsersBooks = ub, Book = b };

WHERE User.UserName IN ('user1', 'user2', 'user3')

WHERE User.UserName = 'user1' OR User.UserName = 'user2' OR User.UserName = 'user3'

foreach(var name in userNames) {
    // Logic to construct final {User:{Books}} JSON object
}

// Logic to construct final {User:{Books}} JSON object
var finalOutput = usersAndBooks.Select(x => new ...);

List<string> userNames = JsonConvert.DeserializeObject<List<string>>(input);