In order to reach this Book, you will have to complete Reluctant Steward. Only afterwards Neloth of Tel Mithryn will let you access the room in which you enchant staffs (screen above). Open the book to move to the dangerous land of Apocrypha.
Learn the Black Book's hidden knowledge
The first level is merely a vestibule - just cross the corridor and activate the switch to open the door to Chapter II. Things get much more interesting there. Go forwards until you reach a big room with two Seekers. Defeat them and get to the upper level of the construction. Use the organic button, quickly run onto the platform which will appear in front of you and immediately jump onto the next one. The pace is very important here - if you fall down, you will have to start everything from the beginning.
Jump across some more platforms and run onto the next "tongue". As you reach the other side of the pit, turn left and take a look inside the treasure chest. Afterwards turn around 180 degrees and approach the book which leads to Chapter III, where more attractions are waiting for you.
Head out of the building and climb onto the roof (proper paths can be found on both sides of the construction). Turn on the switch and head onto the unlocked island. Head further and kill the Lurker inside the room. Afterwards take a look at the small niche illuminated by two plants (screen above). Inside you will find the book-gate to Chapter IV.
After using the Book, you will be almost immediately attacked by a Seeker. Kill him and use the switch in front of you. Afterwards return to Chapter III and unlock the passage to further volumes. You can do it by getting onto the upper level and pressing the next button there (where you fought with a Lurker). It will open a gate in the middle of the room.
Head inside the cell and turn right - at the end of the short, twisting corridor you will find a switch. Use it and enter the newly opened tunnel. Defeat the Seeker who's hiding there and get to the end of the location. There you will find books leading to Chapters V and VI. Use the first one to visit the next level.
In the new location, kill the next Seeker and use the button in front of you. That way you will open a passage to a room with five skill Books which will let you learn the secrets of all schools of magic. After satisfying your hunger of knowledge, return to Chapter III and head to Chapter VI.
Chapter VI to is very small. At the edge of the room you will find Book-gates leading to Chapters VII and VIII. Visit the first one and defeat (as usual, it seems) the Seeker to use the button. Afterwards return to the transition chapter and jump into number VIII.
Right after using the Book, you will be attacked by a Seeker - deal with him and get on the bridge in front of you. When the Lurker gets out of the pond, shoot him from a safe distance or kill in any other way. Use the button in the middle of the room (look out for tentacles!) and cross the long corridor leading to the Black Book. Before taking a look into it, collect the treasures lying around.
Here are the possibilities that Hermaeus Mora give you:
Summons a field of writhing tentacles that poison foes
Freezes the target between Oblivion and Tamriel for 30 seconds, making them immune to all damage
Fully restores your health, magicka, and stamina