The ex libris is an old, forgotten tradition that allows you to mark your books with your name. 
From the Latin ex libris meis, “from among my books”.

And now, let’s see what the dictionaries have to say about it:
In the Larousse : a vignette, drawn or engraved, that bibliophiles paste on the inside cover of their books and which bears their motto or name. In the Robert: An inscription or vignette affixed to a book to indicate its owner.