Librarians hate it, cultural historians love it - under certain circumstances:
mar·gi·na·li·a pl.n. Notes in the margin or margins of a book. [1832, from L. marginalia, neuter plural of marginalis "marginal," from marginis].
"In getting my books, I have been always solicitous of an ample margin; this not so much through any love of the thing itself, however agreeable, as for the facility it affords me of pencilling suggested thoughts, agreements, and differences of opinion, or brief critical comments in general."In deference to librarians, just make sure the books are yours.