I've been thinking about the two page spread, the sort of "atom", or smallest indivisible unit of the photobook, and looking at what people have done with them.
The most obvious remark one can make is that, since the 2-page spread is (basically) what we "look at" each spread needs to function as a unit, somehow or another. You can make a (strong) analogy with composing a picture, since in a very real sense that is what you are doing in laying one of these out. The spreads, of course, have to work with one another, from one spread to the next, from the first one to the last.
The standard approach is to reduce the 2-page spread to a single picture. Usually, this is printed recto with the verso either blank, or carrying accompanying text. A title, a poem, whatever you like. Less often, we see single pictures printed across the gutter. Perhaps a full bleed spread, or something else.
If the author chooses to place more than one photo in a spread, the most common design is one photo recto, another verso, with the two pictures echoing one another in some way. Similar graphical design, similar textures, similar subjects, similar tonalities or colors. This is the easy way to make them "work together" to create a coherent spread.
Szarkowski's The Photographer's Eye is a remarkable example of this. Each spread contains, often, two or more pictures, and is its own little world. Sometimes it's a group of 5 pictures of hands. One places one of Evans's torn movie posters with ruined faces looking to the left, against Lange's Funeral Cortege which features a face in a window, again looking left. There's not a lot going on from one spread to the next, but Szarkowski manages to make each spread amazingly coherent on several levels.
I don't see any particular reason that one could not as well use contrast rather than similarity, but whatever one does, one needs to be cognizant of the rest of the book. If you make verso a high key portrait, and recto a murky architectural study, well, that says something. It's not "together" so you will need it to make sense some other way, in the context of the book.
Indeed, in all cases, one needs to keep in mind the needs of the book. Szarkowski has the luxury of making a survey, so one spread need not particularly related to the next, except in the sense of fitting the larger theme set in each chapter of his book.
It occurs to me that a real tour de force would be to create one theme on the verso pages, and a second theme on the recto pages, while simultaneously making each 2 page spread function on some fashion or another.
About the gutter.
I learned something from Sally Mann's Immediate Family about the gutter and its use. Mostly we consider the gutter a nuisance, a place where content goes to die. Print across the gutter if you must, but try to avoid having important picture elements drop into it.
Mann does something quite different. Most of the book is 2 picture spreads, one recto, one verso, with some strong relationship between the two. Now and then, we get a single photo. Often, it is printed recto with the verso blank. Then we get a handful of single pictures printed across the gutter. Mann can perfectly well just print things recto, she does this a lot, so, what the hell?
The answer is that she's embracing the gutter, which is bloody genius as far as I can tell. I swooned.
If a picture divides neatly into two, she prints it across the gutter with the gutter cutting it at the right spot. Three pictures for the price of one. In a couple of cases, she seems to be, rather, indicating an alternate crop, "take the whole thing, or if you prefer, just take the recto" which is 2 for 1, not quite the bargain, but still a nice price.
Anyways, the big lesson here is that printing across the gutter does not require that the picture be placed symmetrically. The gutter can fall wherever you like, so use it, drop that strong vertical element into the right spot.