__resolveType alternative

Hello, let’s say for example I want to make a search for my site, and I have books and posts, both objects have the same fields (author, title), how can I make it with a union?

On the docs I found this, isn’t it a bit odd to have to check if some fields are in the object? what if I have a lot of objects, what if some objects have the exact same field? Is there a better alternative to resolve a union than this?

const resolvers = {
  Result: {
    __resolveType(obj, context, info){
        return 'Author';
        return 'Book';
      return null; // GraphQLError is thrown
  Query: {
    search: () => { ... }