To reach this location, run to the fortress in Mourningwood and leave it through the gate from which the Hollow Men came. This way you will reach a cemetery . At its end there's fork in the road. The let path will take you to the mansion .
"The Extraordinary Homunculus of Baron Von Orfen"
After reaching the location, go to the summerhouse with sculptures on the left side of the garden . The book is lying on one of the wooden benches .