If you've spent any time knocking around the history of photography, you've probably run into Stieglitz's "Equivalents", these pictures of clouds that he claimed were "equivalent" to some emotional state, or something like that. Minor White went on at some length about this idea of "Equivalent", and seems to have considered it fundamental to photography, or at least to the photography he thought worthwhile. It has something to do with the idea that the photograph represents real things, but that upon viewing it evokes something quite different in the viewer. I think it's more than an evocation, though, it is that the viewer seems the picture as a symbol, an embodiment, of what the viewer feels. There is, in short, something like equivalence, natch.
Anyways, this is one of those concepts that has always felt a bit dodgy to me. I've never seen anyone write about it in anything like a clear fashion, which is rarely a good sign.
Minor White seems to think that, whatever it is, it might be pretty personal, pretty subjective (rather than intersubjective!). Everyone might get something different out. White also seems to subscribe to the theory that only some people can do it, that you might need to be specially attuned, or specially trained in order for an Equivalent to work. I find this notion to always be pretty offensive and useless. Art is bullshit if you need to go to night school to "get" it.
So what am I on about with the Photobooks?
When I look at a picture, I can kind of fill in a world the picture lives in from my imagination and experience. I imagine, at least a little, what's outside the frame, what happened before and after. So I fill out things "horizontally", but there's also a "vertical" filling-in. I react to that "horizontally" expanded view of the picture. I might have some emotional response, I might enlarge my mind on a good day, all that sort of thing. However you put it, even a crappy picture probably ripples outwards in several dimensions in my mind. Whether that's an "equivalent" or not, I can't say, but it certainly happens.
Let's suppose that a functioning "Equivalent" is one where that multi-dimensional space of mental ripples is roughly related to what the photographer had in mind, and that the photograph reads as some sort of symbolic representation of those ripples, rather than simply a thing that kicked the ripples off.
I visualize a photograph as a sort of grappling hook the photographer throws toward that whole complicated mental space, hoping perhaps to hit more or less near a specific spot. Weston seems to want to hit the "sex" part of my brain, which is a bit of a cheat because, let's be honest, it's a huge target.
A portfolio gives the photographer many casts of the hook. The idea can become clearer, there's a much better chance of a portfolio of work doing that "Equivalent" thing, surely, whatever it is.
A photobook is one step farther, since it allows the artist to shape the relationship between the photographs. Consider Keith Smith's notion of the book as, really, a single composite picture built up in the viewer's mind by reading the book, present and complete only after finishing the book. The book, considered this way, is perhaps a single cast of a gigantic grappling hook, ideally so large that it cannot help but get a piece of whatever the artist is aiming for.
Of course, it is also multiple casts of the hook, like a portfolio. So, in a way, it gives the artist several different ways to hit the mark.
It's a rough idea at the moment, but I might almost believe in the possibility of a book (or book-like object) of photos behaving as an Equivalent, in a pretty general and accessible way.