I was about to ask about that, how are / were traditional paper books lined out to prevent this? Surely not by hand. Proprietary software maybe?
Well, before desktop publishing, by phototypesetting [1], before that by hot-metal typesetting [2], before that, by hand. Nowadays, with software like Adobe InDesign, of if you happen to be a CS/math/physics nerd, with LaTeX, which has a famously high-quality layouting engine that utilizes the Knuth–Plass line-breaking algorithm [3]. Indeed it's fairly well known that Donald Knuth created TeX because he wasn't happy with the phototypeset proofs he received in 1977 for the second edition of The Art of Computer Programming, finding them inferior compared to the hot-metal typeset first edition.
[1] https://en.wikipedia.org/wiki/Phototypesetting
[2] https://en.wikipedia.org/wiki/Hot_metal_typesetting
[3] https://en.wikipedia.org/wiki/Knuth%E2%80%93Plass_line-break...
The old linotype machines had visual indicators of minimum and maximum line width, and the operator would make a judgement call with each line. Spacers would then automatically justify the letters. It was all mechanical and amazing.
See http://widespacer.blogspot.com/2014/01/two-spaces-old-typist... for many details.
Books were produced before computers, and with very good typesetting. One difference between websites and books is that theee is a feedback loop with books where somebody ia at looking at the layout and either adjusting the spacing subtly, or even editing the text to avoid problems. Sometimes this is just to ensure that left justified text isn’t too ragged on the right edge, sometimes it’s to avoid river of space rubbing through a paragraph, and sometimes it’s editing or typesetting to avoid orphans.
But text on a page is set for a set layout, and that’s where the web really differs.
In ye olde dayes, indeed by hand. That's why there was often extra space after punctuation. In more mechanized times, the operator has to watch for it. Proofreaders are trained to watch for loose lines, rivers, widows, hyphenation errors, and other spacing problems. Those things will be marked as errors in proof. Even with modern DTP tools, typesetters still have to make a lot of manual corrections. Of course, for print, you're setting for a fixed format. You can do a lot of fine-tuning that a browser can't do on the fly.
The idea of extra space after punctuation (especially periods) being a result of printing technology is a myth. Extra space is present in handwritten documents: go look at the US Declaration of Independence or Constitution for well-known examples. People only started shortening the space between sentences to match the space between words very recently.
Nowdays basically any professionally produced book is made in Indesign. And text wrapping is semi-automated. It's automated but checked for issues and fixed manually. Indesign has two text wrapping algorithms paragraph composer that tries to balance whole paragraphs and line composer that only checks line by line.
Surprisingly in the high-end the less automated line composer is used a lot more. It requires more work but human decisions lead to best results if done properly.