in Editing
Decide once: serial commas, capitals and other house choices
Many style questions have no right answer, only a consistent one. The skill is in choosing early and sticking to the choice.
A surprising number of questions in copy-editing have no single correct answer. Reputable style guides disagree, publishers make different choices, and good writers go both ways. What matters is not which option a book picks but that it picks one and applies it throughout.
The serial comma
The serial comma, also called the Oxford comma, is the comma before "and" or "or" in a list of three or more: "red, white, and blue" rather than "red, white and blue". It takes its name from Oxford University Press, whose house style uses it. The Chicago Manual of Style recommends it too. Many British publishers and newspapers leave it out except where it is needed for clarity.
Both conventions are defensible. The serial comma prevents some ambiguities; omitting it prevents others, and saves a character. A book that uses it in some lists and not in others looks inconsistent. Choose, and apply the choice everywhere, adding a comma in the exceptional cases where it avoids a genuine misreading.
Capitals
Capitalisation is where house choices proliferate. Titles and offices: "the Prime Minister" or "the prime minister", "the King" or "the king". Institutions: "the Church" or "the church". Relatives used as names: "I asked Mum" but "my mum". Seasons, points of the compass, historical periods, deities. Conventions vary from house to house.
The practical approach is to decide a principle, record it on the style sheet, and resolve borderline cases by reference to it.
Quotation marks and dashes
British publishing commonly uses single quotation marks for dialogue, with double marks for quotations within quotations. American publishing typically does the reverse. Many British books follow the American pattern, and both are acceptable, but mixing them is not.
Dashes follow a similar pattern. British typography often uses a spaced en dash for parenthetical breaks; American typography usually uses an unspaced em dash. Again, either is fine, but only one should appear in a given book.
Spelling variants
Within British English, some words have two accepted spellings. The "-ise" and "-ize" endings are the best-known example: Oxford style uses "-ize" for many verbs, while much British publishing prefers "-ise". Other pairs include "judgement" and "judgment", "focused" and "focussed", "towards" and "toward". A book should settle each one.
Every one of these choices is easy to make once and hard to enforce afterwards. A writer who has used both forms of something across a manuscript faces a search-and-check job at the end. A writer who decided at the start, and kept a note, has nothing to fix.
If you are working with a publisher, they may have a house style that settles many of these questions for you. If you are publishing independently, you set the house style, and that is a freedom worth using deliberately.
The reader will not notice any single choice. They will notice when the book seems to have made none.