The book is nothing but references as part of its theme. But Gaddis was extremely well read and was really into esoteric knowledge so a lot of its references are to occult hermetic, pagan, and alchemical treatises, mostly stuff from the middle ages that nobody has heard of.
There's also a ton of literary references to psychoanalysis, Jung, Christian theology, Vedanta, Goethe, Dante, TS Eliot... the list goes on. But to try and get all these things under your belt is asking a lot, and I would argue, that not getting the references is part of the ultimate experience of The Recognitions.
Some stuff that does seem realistic to accomplish reading and that will be worth the time put into it as you pick up The Recognitions are The Divine Comedy, Eliot's Four Quartets and The Wasteland, Goethe's Faust, The Bible, Rilke's Duino Elegies, and Augustine's Confessions. Those are the main canonical texts that Gaddis references.
It hardly seems possible to completely match your knowledge to what The Recognitions references, but The Recognitions doesn't ask that you know anything so much as it demands you know nothing.
