Like all (save for Waking Dreams) Black Books, the one added by this mod also comes with three exclusive rewards you can choose.
Here is the list of the rewards, and the power they grant:

Gift of the Mage:
    Grants the player a great power that can be used to obtain instant-cast version of the Master-level spell he or she knows. Lighting Storm, and the Thrall spells are not included, however.

Gift of the Warrior:
    The player gets a bonus 200 points in unarmed attack, greatly increased stamina regeneration rate, power attack will not consume stamina, and with decreased physical, magical, and poison damage.

Gift of the Thief:
    Grants the player a great power that can be used to obtain a magical key that can open any lock that does not require a key. The key is destroyed upon use, and cannot be dropped. Once used, the player is granted a temporary ability that enables the player to open any lock that is Expert and below.
    The player can hold up to 5 keys at the same time.