The Joy of Tantric Sex with Illustrations for the Advanced Student,by A.Lady
A grimoire stored under extremely plain covers (and lots and lots of ice) in the maximum security cells of the Library. It is rarely accessed for research as this is exactly the sort of thing that older wizards, who in any case prefer big dinners, are prone to getting extremely forgetful about and have to have their minds refreshed as to the underlying concepts involved.
This is of an order with the Compenydyum of Sex Majick by Ge Fordge, where the strict rule is that the grimoire may only be accessed by wizards who are over eighty and, if at all possible, dead.
Given the level of paranoia concerning the origins of Sourcery, and what a rogue wizard has to do to make a Sourcerer (at least eight times. More if you allow for inconvenient daughters), this is only for the best...
A copy, or at least a similar book edited for non-Wizards, exists in the library of the Pharoah in Djelibeybi where it has been used in the theoretical instruction of handmaidens such as Ptraci and the informal education of princes such as Teppic, who in other informal lessons learnt to pick the lock of the cabinet where the manual was kept.
It is probably sheer coincidence that Jane Austen used the nom-de-plume 'by a Lady'.