"Style and Structure are the essence of a book;
great ideas are hogwash."--Vladimir Nabokov
A bunch of poems or stories is not the same as a book of poems or stories. So what makes a book? The question and the answers are all complex, and without one consistent set of rules, but people manage. Take a deep breath, and gather together what you have. There are some approaches that will help you find the book, using various organizations, structures, games, and experiments. In trying these, you will want to look for the house your work will live in best, and into whose living room you want to invite people.
Don't start tailoring your writing to the manuscript too quickly. At some point, that will be appropriate; however, it should not be too soon. Otherwise you will simply be pandering to the product, rather than realizing a truly individuating content. But when you're ready, when the manuscript is ready, when you begin to look outward at what the manuscript can now do for you or for itself, rather than what you can do for it: Then it's time to think about wrapping things up.
Of course all the old writers' adages about a book never being finished, but simply abandoned, will occur to you. That line, incidentally, has been ascribed to various writers, so that who said it may be lost to us. Nevertheless, the point is well taken.
DVICE. Begin by listening to the community of the work. Before anything else, stand back and ask not, "What am I doing?" but rather, "What are these things doing?" Try to distinguish between your intent and a manuscript's content. Intent and content may be two very different realizations.
The material itself will eventually suggest how to organize the manuscript into a book. It is not a big leap to observe that if the poems are about nature, the poems might be grouped by season. If the poems deal with human nature, the organization might be a dialogue, or lack of dialogue. Love poems might constitute a dialogue, with clusters of two poems and occasionally three poems for tension. The cruelty of the world dictates an objective order. All of these are possibilities, of course. But you must do them, not just think them. Thinking these connections is easy, and therefore easy to dismiss. Actually finding them--actually doing the work of finding these connections--is not then so easy to dismiss.