If you are ever lucky enough to get a contract for a book, and if you are lucky enough to have said contract offered by a commissioning editor who still cares enough (and there are fewer of them these days), your book will be read by a third person, known as a copy editor -- sometimes called a line editor.
The copy editor's job is to go through your book and, as a minimum, correct the spelling and make sure that your sentences are not completely incomprehensible and illiterate. A really good copy editor is a great prize. A bad one is an unbelievable pain in the arse.
A good copy editor will spot everything that is questionable or inconsistent, and will ask you, politely, if this is what you really mean, and perhaps it might be better to rephrase this sentence, and is the title of that song really 'I want to hold your hand', or is it 'I wanna hold your hand'? Or should it even be 'I Wanna Hold Your Hand'? And should it be in italics, rather than inverted commas?
A good copy editor (like an old-time typesetter) will know that benefitted is wrong and that benefited is right; what is more, she will know why -- what the rule is, and what the exceptions to the rule are.
Since you ask, the rule about benefited and such, broadly speaking, is that words of more than one syllable, which end with one consonant preceded by one vowel, double the final consonant on adding suffixes such as -ed, or -ing, but only if the last syllable of the word is stressed. Hence occur becomes occurred, begin becomes beginning, and so forth, because the emphasis falls on the second syllable. Benefit, on the other hand, has the emphasis on the first syllable -- hence benefited, not benefitted.
All perfectly simple really, isn't it? A fuller explanation is given in Fowler's Modern English Usage; I am using the third edition.
Mind you, since this is the English language that we are talking about, there are exceptions to the rule stated above, notably words ending in w, x, or y. (Guffaw becomes guffawed.) Also words ending in -l generally double the last consonant whether stressed on the last syllable or not. (Libel and libelled; annul and annulled.)
And, what's more, the Americans screw up all these rules by spelling everything differently anyway. I don't know, you give people a perfectly sensible language, and what do they do? Go their own sweet way, as per usual.
Back to copy-editing. As an example of how a copy editor's mind works, I once had a lady ring me up and point out that I had spelt the word artist (referring to a performer) in two different ways. Sometimes it was artist, and sometimes it was artiste. Did I really intend that?
As it happened, I did really intend it. I used the word artist when writing as the author/narrator. The word artiste was used, I said, only in dialogue, and only by one person, who was a bit of a poseur. But I was grateful to the copy editor for reading the book sufficiently alertly to have noticed.
A bad copy editor will cause you endless grief. Some of them seem to have delusions of grandeur. There was, for instance, the case of Joan Aiken, now deceased. Shortly before her death, Joan wrote an account in The Author (journal of the UK Society of Authors), in which she described how she had been treated by (as it happens) an American copy editor.
Background: Joan had written books for thirty years; she has 510 entries on Amazon. Such a writer, you might think, ought to be assumed to know what she is doing. But no. This copy editor had gone through the whole of Joan's last book and had 'improved' it.
Joan had written: 'Hark at the wind,' shivered George.
The copy editor substituted: 'Listen to the wind,' said George with a shiver.
And so on. On every goddam page. Hellfire, I wouldn't have done that to the ten-year-old boys I used to teach.
Another case known to me concerns a book (not his first) written by a 70-year-old businessman, about the management of companies. The copy editor (British, and evidently feminist) had gone through his text and made it politically correct. Where the author had put Chairman, the copy editor wanted Chairperson.
The author, not surprisingly, blew a gasket. He pointed out, with gritted teeth, that there is not a single company quoted on the UK Stock Exchange which has a 'Chairperson'. They all have Chairmen, thank you very much. (Mrs GOB, by the way, is Chairman of the local flower club, the membership of which is 100% female, and there is no move afoot to change the name of her office.)
Fortunately, tales of disastrous dealings with copy editors are not common. For the most part, the copy editors do a magnificent job, for next to no money, and precious little thanks. And what a hell of a job it is. I've done some, and I know. It's hard grind, and badly paid.
So if and when you get your book under way, hope that you will be given the privilege of being copy-edited, and, even though you may not wish to adopt the suggested changes, be glad that someone read the book, probably more carefully than anyone else will, and took the trouble to check things with you.
All of the above is prompted by an article on the Book Standard in which Adam Langer pays tribute to these oft-forgotten heroes (actually more often heroines), and interviews a few of them about their working practices. Well worth reading.
Another excellent web site which any serious writer ought to look at now and again is Bill Walsh's The Slot. Bill is the Copy Desk Chief, Business Desk, at the Washington Post, and therefore knows a fair bit about editing copy. His site is admirably self-explanatory, and gives details of his book, Lapsing into a Comma.
By the way, I normally use Oxford reference books when doubtful about how to spell or punctuate. And Oxford suggests the we should write copy editor (noun, two words) and copy-edit (verb, hyphenated). Adam Langer's article uses neither space nor hyphen, but prints copyeditor and copyedited. Bill Walsh follows Oxford (which may be the same, for all I know, as Chicago, or wherever). You are free, I guess, to take your pick in relation to these and similar options. But whichever you choose, have a good reason for doing so; and be consistent.