On the whole, because I tend to throw out any poem that doesn't seem to me to do the job, I include whatever is left on my desk when it comes to making a book. One principle, not a very clever one but one I've used, one principle of organisation is simply to alternate long and short, so as to give relief of a kind, to alternate light and heavy to some extent, although some very light poems will not mix with some serious ones. I seldom gather poems with adjoining themes into a section, but it has been done for me. My friend William J Smith is very good at putting books into shape for other people and he's done it for me twice now I think, not only gathering poems into sheaves or sections but also giving them section titles taken out of the poems. There's something to be said for the section title and it gives you an opportunity for a nearly blank page.
I'm not sure what else I can say about that. I do include a lot of translations in my books and it's not because I think they are poems by me, but because I think they are worth reading. I think my practice has been to put the shorter translations from other poets in the main body of the book, perhaps also on a principle of relief, and then in the case of something sustained, to put it into a final section or appendix. One of my books ends with a canto of Dante and the introduction to Molière's play, Amphytrion, and both of those things are much too long to have tossed into the central body of the book. I'm not sure that I can refine any further on the science of book assembly. It's important to start with a good one, I know that.