On John McIntyre's blog You Don't Say, I recently learned about a site where you can search 43 different stylebooks at once. These run the gamut from the recommendations of the American Anthropological Association to those of the World Health Organization, by way of The Economist Style Guide, Jack Lynch's Guide to Grammar and Style, the Oregon Department of Human Services, and The Videogame Style Guide and Reference Manual.
The site is OnlineStylebooks.com, and as John notes, "This should help you to learn how to live with inconsistency". Which is ironic, since the purpose of stylebooks is to help achieve consistency.
The site's "About" page explains:
OnlineStylebooks.com is owned and operated by Mary Beth Protomastro, who has been reading stylebooks since high school. The founder of Copyediting newsletter, she is the copy chief of More magazine and was editor of the Time magazine stylebook. OnlineStylebooks.com is not affiliated with any of those publications.
Mary Beth created OnlineStylebooks.com to help copy editors (including herself) quickly consult a variety of style guides.
Surveying this explicit variety of sources may help to avoid an otherwise-natural confusion. These stylebooks are not about the nature of (the formal written variety of) the English language: each of them documents (aspects of) one organization's policy about how to represent this language in writing, typically covering a limited set of cases that are both reasonably common and somewhat variable in general practice.
Some people are tempted to treat policies of this kind by analogy to the theological differences among religious sects. Believers are convinced that one set of policies is (or should be) God's Truth, with the others to be consumed in the fires of hell, while skeptics think that they're all just different forms of nonsense.
Both of these attitudes seem to me to misread the situation. Once you decide, for whatever reason, that representational consistency is a Good Thing, then you need to deal with the Long Tail of Linguistic Complexity. It's tempting to think that there are a few basic axioms from which the Right Answer could always be logically derived for any question of linguistic analysis — and perhaps some day a future linguistic Peano will give them to us. But as things stand, questions of linguistic representation are more like common law than like set theory. The only known way to achieve reasonably consistent results is to reason from a very long list of precedents, which is always in the process of gradual development, with occasional major revisions. This rational catalogue of worked examples is meant to be consistent with a hierarchy of more general principles, but it's not reducible to them.
You can see examples of this process in the "style guides" that are developed by serious projects in large-scale linguistic description, whose authors typically desire consistency because it serves the needs of current methods in machine learning. A traditional, simple, but useful example is the Penn Treebank Tagging Guide.
Here's one small piece of that work (p. 18):
The indefinite pronouns naught, none and compounds of any-, every-, no- and some- with -one and -thing should be tagged as nouns (NN), not as pronouns (PRP). The sequence no one should be tagged no/DT one/NN; in its hyphenated form no-one, it should be tagged NN.
The basic choice made here may or may not be the best one. In fact, the first sentence seems ironically self-contradictory, in that it gives a list of what it calls "indefinite pronouns", and then tells us to tag them as nouns. But the point is not to settle the theoretical question one way or another, but rather to ensure a consistent treatment. If you believe that someone/NN should really be someone/PRP instead, you can go through the Treebank and do a global replacement. But if someone is sometimes tagged one way and sometimes another, it becomes more difficult to tell whether a proposed tag sequence is right or wrong, and therefore more difficult to develop automated taggers.
It's somewhat less obvious to me why representational consistency is a Good Thing in ordinary writing. There might be a compelling argument from reading speed — appearances to the contrary, the catch-as-catch-can orthography of Shakespeare's day may be so much less efficient to read that it's worth the trouble to learn to spell and punctuate in the modern more-or-less consistent fashion. Whatever the truth of this idea — and I'm not sure that it's ever really been tested — I suspect that a less utilitarian motivation was historically dominant, something to do with a sense of overall order and regulation.
Similarly, I'm not entirely sure why organizations and publications want to impose a house style, which imposes a consistent set of choices in cases where overall orthographic standards are open to various alternative outcomes. I'm not saying that having a house style is a bad thing, or arguing against it, I'm just sightly puzzled about why people care that all the articles in notable publication X should hyphenate and abbreviate according to one set of rules, while all the articles in esteemed publication Y consistently do it a different way.
But publishers and editors and (to some extent) readers clearly do care. And given the decision to care, you've got to find some way to deal with that Long Tail. Hence the stylebook — and since different organizations are concerned with different pieces of the Long Tail, the profusion of stylebooks is as much a matter of extending coverage to new areas as of sectarian splitting on the treatment of old ones.
There are several logical reasons to use Ms. Protomastro's site. Perhaps you aren't bound to follow any particular set of guidelines, but would like to be sure that your choices are somewhere in the stylistic mainstream. Or maybe the crucial benefit is access to the specialized recommendations of organizations like NASA or the Massachusetts Supreme Judicial Court. Or maybe you just like to read style guides.
I don't really fall into any of these categories, most of the time, but I found the site fascinating anyhow.