books-api | GraphQL books API made using Spring Boot and DGS | GraphQL library

It's hard to understand exactly what goes wrong but I would suggest removing the obvious bugs you have in (at least?) 4 places where you do not use your completion handler.

The first one is

So you can use closures as callbacks in your HomeViewController . Let me explain.

First Define the closure expression shown below, globally. The point of having it globally is that you can reuse the same expression throughout your app. I've set it as a typealias for easy reuse.

Your getBook() method doesn't return anything, therefore, this.books will always be null.
As you're using fetch here, you can return the promise in your getBook() method allowing you to perform any action with the data retrieved.

The call to the API is failing, and you're not using functions.https.HttpsError correctly. According to the linked API documentation, the first argument to the constructor must be a FunctionsErrorCode type object, which must be one of the following strings:

"ok" | "cancelled" | "unknown" | "invalid-argument" | "deadline-exceeded" | "not-found" | "already-exists" | "permission-denied" | "resource-exhausted" | "failed-precondition" | "aborted" | "out-of-range" | "unimplemented" | "internal" | "unavailable" | "data-loss" | "unauthenticated"

These strings each map back to an HTTP status code, so you'll have to decide what you're actually telling the client. You probably want "internal" if you just want to indicate a failure that can't be resolved.

The problem is in the component.html line where you have {{item.volumeInfo.imageLinks.thumbnail}} imageLinks is undefined and you can't get thumbnail of undefined. you have to check if item.volumeInfo.imageLinks != undefined before you get the thumbnail.