I just wanted to get this off my chest.

If you want to separate clauses in a sentence, received wisdom, grammarians, and most style guides, recommend using an em-dash which abuts the last word of the first clause and the first word of the second, as in, “When Todd offered him another nip—this was around two o’clock—Tom refused.”* This is fine for printed texts where someone with layout skills has worked to ensure the text is spaced properly and lines flow properly (also, where the width of the page, font style and font size, are guaranteed for the compositor) but, in ebooks, this style of punctuation can cause layout problems since ebook text is laid out automatically using an algorithm that attempts to adjust on the fly for variations in screen size, font style, font size, and hyphenation rules. This is because the two words joined by the em-dash are treated as a single unit (e.g. “o’clock—Tom”) and very long “words” can ensue, making attractive layout difficult for the algorithm. Hyphenating one of the conjoined words creates a messy result and breaking the line at the em-dash can lead to confusion as a word followed by an em-dash is a typical device to indicate an interrupted sentence (usually in speech – see below).

The best way to fix this, in my view, is to leave a gap before and after the dash. Thus, our sentence becomes, “When Todd offered him another nip — this was around two o’clock — Tom refused.” This enables the algorithm to break lines in appropriate places and to improve the layout overall. Most writers, until they are taught not to, use this technique quite naturally in their prose, anyway.

My feeling is that the em-dash was originally chosen to break up clauses because it is long enough that embedding it in a sentence with no space around it, would still leave the words legible. I believe that an em-dash surrounded by space actually has too much prominence. So it is my preference is to use a shorter, en-dash, in my own writing. (A hyphen would be more convenient as it is right there on the keyboard but I find that, in most font styles, hyphens are too short to achieve the required salience.)

The suggested scheme breaks down in the case of interrupted sentences, e.g. “I know – “* Here, the page layout algorithm might choose to break the line before the dash or after it, both of which would obviously be wrong. So, I use the dash with no spaces around it, as in, “I know–” (or “I know—” with an em-dash), which gives the proper formatting behaviour.

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*Unsurprisingly, some publishers have already noticed and solved this problem. The quotes I have been using are taken from Hodder & Stoughton’s Kindle edition of Stephen King’s, Finders Keepers. This publisher uses the scheme I recommend, only with em-dashes (publishers are such traditionalists). Thus, em-dashes with spaces are used to separate clauses and em-dashes without spaces are used for interrupted sentences.