var deserialized = JsonConvert.DeserializeObject<ImportAuthorDto[]>(jsonString);

            var authors = new List<Author>();

            StringBuilder sb = new StringBuilder();



            foreach (var dto in deserialized )
            {
                var mailExists = context.Authors.FirstOrDefault(m=>m.Email==dto.Email);

                if (!IsValid(dto) || !dto.Books.All(IsValid) || mailExists!=null)
                {
                    sb.Append(ErrorMessage);
                    continue;
                }
                var books = new List<int>();

                foreach (var bookDto in dto.Books)
                {
                    var book = context.Books.FirstOrDefault(b=>b.Id==bookDto.Id);
                    if (book==null)
                    {
                        continue;
                    }

                    books.Add(bookDto.Id);
                }

                var author = new Author
                {
                    FirstName = dto.FirstName,
                    LastName = dto.LastName,
                    Phone = dto.Phone,
                    Email = dto.Email,
                    AuthorsBooks = books.Select(b => new AuthorBook
                    {
                        BookId = b
                    }
                        ).ToArray()

                };
                authors.Add(author);
                string authorName = $"{author.FirstName} {author.LastName}";
                sb.AppendLine(string.Format(SuccessfullyImportedAuthor,authorName,author.AuthorsBooks.Count));

            }

            context.AddRange(authors);
            context.SaveChanges();

            return sb.ToString();
        }

-----
 public class ImportAuthorDto
    {
        [MinLength(3), MaxLength(30), Required]
        public string FirstName { get; set; }

        [MinLength(3), MaxLength(30), Required]
        public string LastName { get; set; }

        [EmailAddress]
        [Required]
        public string Email { get; set; }

        [RegularExpression(@"^[0-9]{3}-[0-9]{3}-[0-9]{4}$"), Required]
        public string Phone { get; set; }

        public ICollection<ImportBookDto> Books { get; set; }
    }
-----
 public class ImportBookDto
    {
        [Required]
        public int Id { get; set; }
    }