A Phantasmonomicon is a book that holds Phantasm pages.
It is available from Umbrion's Keep Tower after donating a pinch of catsbane, completing 3 short tests, and paying 1000c.
Speak to Cerider in Umbrion's tower keep while holding a pinch of catsbane.
Once you've donated the catsbane to Cerider, speak to them again to begin.
Test 1 Identify the phantasm out of the fires by bumping in to it.
Test 2 Identify the 3 phantasms out of the 6 fires by bumping in to it.
Test 3 Navigate a maze filled with a mixture of phantasm and real fires.
Once all 3 tests have been completed speak to Cerider to buy a Phantasmonomicon for 1000c
In all 3 tests the "real" fires will block your movement.
Test 2 appears to be the hardest as the phantasms are not in the same place every time so it's blind luck as to whether you pick the correct 3 fires without getting any wrong.