Book을 Setter를 열어둬서 dto정보를 받지 않고 빌더 패턴을 사용해서 다음과 같이 구현해보았습니다

상속 받은 엔티티는 @Builder를 사용하는게 아니라, 다음과 같이 @SuperBuilder로 자식, 부모 엔티티 모두에 선언해줘야 한다고 합니다.

<Book 엔티티>

package jpabook.jpashop.domain.item;

import jakarta.persistence.DiscriminatorValue;
import jakarta.persistence.Entity;
import lombok.*;
import lombok.experimental.SuperBuilder;

@Entity
@DiscriminatorValue("B")
@Getter
@SuperBuilder
@AllArgsConstructor
@NoArgsConstructor
public class Book extends Item {

    private String isbn;
    private String author;
}

<Item 엔티티>

package jpabook.jpashop.domain.item;

import jakarta.persistence.*;
import jpabook.jpashop.domain.Category;
import jpabook.jpashop.exception.NotEnoughStockException;
import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.experimental.SuperBuilder;

import java.util.ArrayList;
import java.util.List;

@Entity
@Getter
@SuperBuilder
@AllArgsConstructor
@NoArgsConstructor
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Item{

    @Id
    @GeneratedValue
    @Column(name = "ITEM_ID")
    private Long id;

    private String name;
    private int price;
    private int stockQuantity;

    @ManyToMany(mappedBy = "items")
    private List<Category> categories = new ArrayList<>();

    /*비즈니스 로직*/
    //todo: stock 증가
    public void addStock(int quantity) {
        this.stockQuantity += quantity;
    }

    //todo: stock 감소
    public void removeStock(int quantity) {
        int restStock = this.stockQuantity - quantity;
        if (restStock < 0) {
            throw new NotEnoughStockException("need more stock");
        }
        this.stockQuantity = restStock;
    }
}

<ItemController 클래스>

package jpabook.jpashop.controller;

import jakarta.validation.Valid;
import jpabook.jpashop.domain.item.Book;
import jpabook.jpashop.service.ItemService;
import lombok.RequiredArgsConstructor;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;

@Controller
@RequiredArgsConstructor
public class ItemController {

    private final ItemService itemService;

    @GetMapping("/items/new")
    public String createForm(Model model) {
        model.addAttribute("form", new BookForm());
        return "items/createItemForm";
    }

    @PostMapping("/items/new")
    public String create(@Valid @ModelAttribute("form") BookForm form, BindingResult result) {

        if (result.hasErrors()) {
            return "items/createItemForm";
        }

        Book book = createBook(form);
        itemService.saveItem(book);

        return "redirect:/";
    }

    //todo: dto 정보를 받아서 엔티티를 만드는 단순 작업은 컨트롤러에서 처리
    private Book createBook(BookForm form) {
        return Book.builder()
                .name(form.getName())
                .price(form.getPrice())
                .stockQuantity(form.getStockQuantity())
                .author(form.getAuthor())
                .isbn(form.getIsbn())
                .build();
    }
}

이렇게 하고, Jpashop의 상품등록에 들어가서 데이터를 입력하면 정상적으로 DB에 데이터가 저장됩니다.