"that famous library where the treasure is kept"

Milton's notebook containing the draft of Lycidas (which Charles Lamb had looked at)  may be found in  The Wren Library  at Trinity College, Cambridge.

Interior of Wren Library, Trinity College, Cambridge
Creative Commons Attribution Share AlikeInterior of Wren Library, Trinity College, Cambridge - Credit: Andrew Dunn, Wikimedia Commons
Exterior of Wren Library, Trinity College, Cambridge
Creative Commons Attribution Share AlikeExterior of Wren Library, Trinity College, Cambridge - Credit: Andrew Dunn, Wikimedia Commons