@Entity
@Indexed
public class Book {

    // ...

    private BookEmbeddable emb;

    @Embedded
    @IndexedEmbedded
    public BookEmbeddable getEmb() {
        if (emb == null) {
            emb = new BookEmbeddable();
        }
        return emb;
    }

    public void setEmb(BookEmbeddable emb) {
        this.emb = emb;
    }
}

@Embeddable
public class BookEmbeddable {

    private Map<Long, String> categories;

    @ElementCollection
    @IndexedEmbedded
    @MapKeyColumn
    @Field(bridge = @FieldBridge(impl = CategoriesBridge.class))
    public Map<Long, String> getCategories() {
        if (categories == null) {
            categories = new HashMap<Long, String>();
        }
        return categories;
    }

    public void setCategories(Map<Long, String> categories) {
        this.categories = categories;
    }
}