What use are a book, a camera, and a knife to the diver ?

In Adrienne Rich's poem "Diving into the Wreck," the three items in your question—the book, the camera, and the knife—are all mentioned at the beginning of the poem. The first part of the opening stanza reads:

First having read the book of myths,
and loaded the camera,
and checked the edge of the knife-blade,
I put on the body-armor of black rubber
the absurd flippers
the grave and awkward mask. (lines 1–6)

In these lines, the three items are all part of the speaker's preparation for her "dive." She has "read the book of myths," so she is informed of all of the falsehoods and origin stories that she apparently needs to successfully complete her mission. The camera is "loaded," indicating that the speaker plans to take pictures as evidence of something during her dive. She also has sharpened "the edge of the knife-blade." This suggests that she is expecting to have to defend herself or to cut something loose. Her "body-armor" supports the former idea. At the end of the stanza, the speaker asserts that she must undertake her mission "alone."
After describing her experience exploring the wreck in vivid detail, the speaker returns to those three items in the closing stanza:

We are, I am, you are
by cowardice or courage
the one who find our way
back to this scene
carrying a knife, a camera
a book of myths
in which
our names do not appear. (87–94)

The poem has emphasized that the speaker and anyone else who would brave the wreck must do so alone. Here, she reiterates that one who would dive would have to "find [her] own way." Those brave people would carry the same three pieces of equipment, but here, the speaker puts a twist on the description that we saw in stanza one. This book is one "in which / our names do not appear." This implies that the speaker can explore the wreck after reading the book of myths, and she can discover the truth, but she will still not be part of the "official history." As a feminist poet, Rich often writes about the oppression of women or gender inequality in society. This poem does it rather subtly, but this sentiment that closes the poem suggests the inferior placement of women in society throughout history.