The closest to what you originally asked¹ for is something like this:

Copy   Class<?> clazz = Book.class;

   Object someObject = new Book(...);
   
   if (clazz.isInstance(someObject)) {
      Object instanceOfClazz = clazz.cast(someObject);
      // ...
   }

(Note that this Q&A - Is there something like instanceOf(Class<?> c) in Java? - directly answers the question in the your question title.)

But you objected that the result of the cast call:

"would lose all the methods of the Book class".

First of all that is not technically correct. The object that instanceOfClazz refers to is still and will always remain a Book. It still has and will always have all of the methods of a Book.

But you do have a point (I think). You cannot use the cast call's result effectively in the above code because you assigned it to a variable whose type is Object. And you cannot call Book methods via a variable / value whose static type is Object.

If instead we had written this:

Copy   Class<Book> clazz = Book.class;

   Object someObject = new Book(...);
   
   if (clazz.isInstance(someObject)) {
      Book book = clazz.cast(someObject);
      // call some `Book` methods.
   }

Now you can call Book methods on book. But the gotcha is that we had to write Class<Book> instead of Class<?> (or Class) to make that work. So we have ended up wiring the class name Book into the code anyway. Ooops!

The fundamental problem is that Java code cannot make regular method calls on an object unless the compiler knows what its type is ... and that it has those methods. If you want to invoke Book methods in cases where the static type of the variable is (say) Object, then you have to use reflection to do it. Something like this:

Copy  if (clazz.isInstance(someObject)) {
      Method method = clazz.getMethod("setNosPages", int.class);
      method.invoke(someObject, 99);
  }

and deal with the various exceptions that that might throw.

Basically, you refer to the class and methods by name in the code, or you use reflection.

"I need to assign the object/value parsed from a json to a field of type Book (or other types accordingly) of another class."

It won't work. Java is a statically typed language². Think of another way to solve the problem. For example, use reflection to assign to the field.

^{1 - That is ... what the words of your question originally asked!
2 - OK ... that is over-simplifying.}

The old way is to test instanceof and them cast to the desired type:

if(this.areas.get(i) instanceof Habitat) {
  Habitat area = (Habitat) this.areas.get(i);
// rest of the if block