Tipped-in describes a leaf, plate, signature page or label attached into a book with a thin line of glue along one edge, rather than sewn in with the rest of the pages. Publishers used it to add illustrations, errata slips or signed pages after a book was printed.
How to spot it
Open the book gently at the page and look along the inner margin. A tipped-in leaf is held by a narrow strip of glue and often sits slightly proud of the pages around it, or lacks a matching leaf on the other side of the fold. Signed tipped-in pages are common in special and limited editions, where the author signed loose sheets that were then glued in.
What it does to value
A tipped-in signature is a real signature, but some collectors value it a little below a signature on the title page, because it was not signed in that particular book. A tipped-in plate or errata slip that belongs to the book should be present, and its absence lowers value. Loose or detached tipped-in leaves are a condition fault.
See signed and inscribed books and limited and special editions for how tipped-in pages appear in signed and special editions.