"to a certain extent Milton is right - the mind is its own place and in itself can make a Heaven or Hell and so forth"
was a seventeenth century English poet, most famous for his book-length work Paradise Lost
which is repeatedly referenced in The Secret History. Paradise Lost
is an epic poem, written in blank verse, which recounts the Biblical story of 'The Fall': Adam and Eve's temptation and expulsion from the Garden of Eden.
The quote Richard mentions is from Book 1.1.254:
The mind is its own place, and in itself
Can make heaven of Hell, and a hell of Heaven.
The whole of Paradise Lost can be read online here.