I need Apollo-GraphQL Expert's help!

Hi! I started to study apollo-graphql. I created one project and it has two modules such as book and user.(I attached project structure image.)[enter image description here][1] Now I use two schemas(book and user), but I want to integrate one. Because in server.js ApolloServer use only one schema and resolver. I attached schema, model and resolver files. If you have experience in it, please help me. Thanks John Davis

    /* Book.js */
    var mongoose = require('mongoose');

    var BookSchema = new mongoose.Schema({
      id: String,
      isbn: String,
      title: String,
      author: String,
      description: String,
      published_year: { type: Number, min: 1945, max: 2020 },
      publisher: String,
      updated_date: { type: Date, default: Date.now },
    });
    module.exports = mongoose.model('Book', BookSchema);


    /*bookResolvers.js*/
    const Book = require('./Book');

    const resolvers = {
      Query: {
        books: () => Book.find({})
      },
      Mutation: {
        addBook: (parent, book) => {
          const newBook = new Book({isbn: book.isbn, title: book.title, author: book.author, description: book.description, published_year: book.published_year, publisher: book.publisher});
          return newBook.save();
        }
      }
    };

    module.exports = resolvers;

    /*bookSchema.js*/
    const { gql } = require('apollo-server-express');

    export const typeDefs = gql`
      type Book {
        _id: String,
        isbn: String,
        title: String,
        author: String,
        description: String,
        published_year: Int,
        publisher: String,
      },
      type Query {
        books: [Book]
      },
      type Mutation {
        addBook(isbn: String!, 
          title: String!,
          author: String!,
          description: String!,
          published_year: Int!,
          publisher: String!,
        ): Book
      }
    `;


    /*User.js*/
    var mongoose = require('mongoose');

    var UserSchema = new mongoose.Schema({
      email: String,
      password: String
    });

    module.exports = mongoose.model('User', UserSchema);

    /*userResolver.js*/
    const User = require('./User');

    const resolvers = {
      Query: {
        login: async (parent, params) => {
            const user = await User.findOne({email: params.email}).exec();
            console.log(user);
            return user;
        }
      },
      Mutation: {
        register: (parent, params) => {
          const newUser = new User({ email: params.email, password: params.password });
          return newUser.save();
        }
      }
    };

    module.exports = resolvers;

    /*userSchema.js*/
    const { gql } = require('apollo-server-express');

    const typeDefs = gql`
      type User {
        _id: String,
        email: String,
        password: String
      },
      type Query {
        login(email: String, password: String): User
      },
      type Mutation {
        register( email: String, password: String): User
      }
    `;

    module.exports = typeDefs;

    /*resolvers.js*/

    /*schema.js*/

    /*server.js*/
    import schema from "./schemas/schema"
    import resolvers from "./schemas/resolvers"

    const express = require('express');
    const { ApolloServer } = require('apollo-server-express');
    const mongoose = require('./config/database');

    const server = new ApolloServer({ schema, resolvers });
    const app = express();

    server.applyMiddleware({ app });

    app.listen({ port: 4000 }, () =>
      console.log(`Server ready at http://localhost:4000${server.graphqlPath}`)
    );


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