Ideally, when you begin writing your thesis, you would have all the necessary books at home, both new and antique (and you would have a personal library, and a comfortable and spacious working environment where you can divide the books that you will be using into different piles, arranged on many tables). But this ideal condition is very rare, even for a professional scholar. In any case, let us imagine that you have been able to locate and purchase all the books.
If the book is yours and it does not have antiquarian value, do not hesitate to annotate it. Do not trust those who say that you must respect books. You respect books by using them, not leaving them alone. Even if the book is unmarked, you won’t make much money reselling it to a bookseller, so you may as well leave traces of your ownership.
Umberto Eco
1932 - 2016