From <a href="http://www.tappinbookmine.com/glosdefn.htm">this page</a>: Something is said to be "tipped in" a book when it has been attached to a page of the book by its corners only. The point is that the entire back surface is NOT slathered with glue and firmly attached. When the entire, or most of the back surface of the object it attached, it is "pasted in" or perhaps "glued on". If an entire edge is attached, it may still be referred to as "tipped in". The usual means of attachment is glue or paste, however it is conceivable some other means may be used. The object "tipped in" is nearly always of paper, as an illustration, letter, paper sample or such. Illustrations "tipped in" lend an aura of quality to books, as it is perceived to be a more expensive and finer way of including illustrations.