Search function for books
Since your search is only by 1 parameter, a map&lt;string,Book&gt; would work a lot better than a vector.

map&lt;string,Book&gt; mBooks;
string LibrarySystem::BookDataOutput(string bookId)
{
    Book book = mBooks[bookId];
    string data = 
        "Book author: " + book.bookAuthor + "\n" +
        "Book name: " + book.bookName + "\n" +
        "Book type: " + to_string(book.bookType) + "\n" +
        "Book Id: " + book.bookID;
    return data;
}