The speaker is describing the pleasure of a book, and the poem does it through a string of odd, playful comparisons rather than plain statement. The book is first an "oyster busy as a rose", a strange mix of images. An oyster is closed and hidden, while a rose is open and unfolding, so the book seems to be both: a shut object that is secretly full of life and growth. Then it becomes "a pleasant room", a place you can step into and stay in.
The next image is the surprising one. The room is "bigger than a broke-open safe," which suggests treasure that has been cracked open and spilled out, though the book is even larger than that. The final line, "a great large book," sounds almost childlike and flat after all the fancy comparisons, as if the speaker has run out of clever words and simply admires the thing.
The mood is delighted and a little dreamlike. The title, "Bibliophile," tells us the speaker loves books, and the poem's logic feels like the logic of that love.