interesting description of this Library
by Montaigne:

Le 6 de Mars [1581] je fus voir la librerie du Vatican
qui est en cinq ou six salles tout de suite. Il y a un
grand nombre de livres ataches sur plusieurs rangs de
pupitres; il y en a aussi dans des coffres, qui me
furent tous ouverts; force livres ecris a mein et
notamment un Seneque et les Opuscules de Plutarche. J'y
vis de remercable la statue du bon Aristide[407] a tout
une bele teste chauve, la barbe espesse, grand front, le
regard plein de douceur et de mageste: son nom est
escrit en sa base tres antique....[408]

Je la vis [la Bibliotheque] sans nulle difficulte;
chacun la voit einsin et en extrait ce qu'il vent; et
est ouverte quasi tous les matins, et si fus conduit
partout, et convie par un jantilhomme d'en user quand je
voudrois[409].

Sixtus IV. intended the library attached to the Holy See to be of the
widest possible use. In the document appointing Demetrius of Lucca
librarian, after Platina's death, he says distinctly that the library has
been got together "for the use of all men of letters, both of our own age,
or of subsequent time[410]"; and that these are not rhetorical
expressions, to round a phrase in a formal letter of appointment, is
proved by the way in which manuscripts were lent out of the library,
during the whole time that Platina was in office. The Register of Loans,
beginning with his own appointment and ending in 1485, has been printed by
Muentz and Fabre, from the original in the Vatican Library[411], and a most
interesting record it is. It is headed by a few words of warning, of which
I give the general sense rather than a literal translation.

Whoever writes his name here in acknowledgment of books
received on loan out of the Pope's library, will incur
his anger and his curse unless he return them uninjured
within a very brief period.

This statement is made by Platina, librarian to his
Holiness, who entered upon his duties on the last day of
February, 1475[412].

Each entry records the title of the book lent, with the name of the
borrower. This entry is sometimes made by the librarian, but more
frequently by the borrower himself. When the book is returned, Platina or
his assistant notes the fact, with the date. The following entry, taken
almost at random, will serve as a specimen:

Ego Gaspar de Ozino sapientissimi domini nostri