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This question especially affects (law) books for which a new edition is published every 2 or 3 years. Assume that:
the authors didn't detail or record the changes between the older and newest edition.
Rereading the older and newest editions is already time-consuming. Comparing page-by-page is too inefficient and untrustworthy (your eyes might easily miss something).
Both editions may not exist in a computerised format and be searched on a computer.
The easiest way would be to contact the authors.
You said that comparing page-by-page is too inefficient and untrustworthy. However, you don't need to read every word. You can just compare the first 2-3 words of each paragraph, as well as the number of lines of each paragraph.
If I were doing this, I wouldn't read the new information immediately. I would highlight it and continue comparing the beginning of the next paragraph and the number of lines in it. After finishing the whole chapter, or the whole book, I would then go back and read the highlighted chapters.
To cut the task in half, you and a friend / colleague can each tackle half of the book.
(UPD. Disregard the first paragraph - haven't read a single old-fashioned paper book in the last decade, I missed the fact that they are printed on both sides of the pages. Looking through 4 overlaying texts at once would probably turn them into an unintelligible mess. The idea with a book scanner may still work, though...)
If those books have strict and unchanging requirements about fonts, margins size etc., and if the content doesn't change much between editions, would it be possible to compare by overlaying corresponding pages on top of a glass panel with a lamp underneath? Sort of similar to how animated movies were drawn before the computers, but in reverse? You would notice immediately if the pages are identical. If there's a change in the middle of one of the pages, you would also see it. Of course, then the rest of that page (or even chapter) wouldn't match, even for identical texts, but you might be able to shift one of the pages to align the next paragraphs and continue.
If the idea works, you may try a more technologically advanced variation of it using a book scanner (not necessarily that model; then you compare two semi-transparent page images on your computer screen instead, may be able to automate that some more, e. g. cut out paragraphs, highlight significant differences, etc.). If it doesn't, then maybe add some sort of OCR into the process?
Macro to micro hack (to locate content change):
For this technique of content comparison to work most easily, the two editions in question should have the same publisher (same editor, same typographer, same printer, and same book binder.) to ensure as many processes used to publish both editions are the same. Check.
Updating references: Rather than burden a law office with selectively replacing a volume or two periodically, whole editions are provided for replacement to ensure error prevention (by omission), consistency, and continuity. Inevitably, there will be changes to content and preservation of the law firm image depends on public perception (uniformity as an indicator of control, competence) as well as excellent jurisprudence performance.
For each publication volume, compare its updated version to see if changes are obvious before you begin to locate the minuscule changes that are inevitable. Here are steps in a flow chart working from the mechanical layout to locate specific content to compare:
Recap: Whenever you find any anomaly between the different edition volumes (macro), you can begin to check more closely (micro) to determine the specific difference in the updated version.
Good luck.
Reading the entire new book will indeed be laborious. Fortunately, comparing 2 documents & finding similarities (& dissimilarities) between them is something Plagiarism Checking Tools are very good at.
Take a look at: https://app.copyleaks.com/text-compare
Here is another tool you can try: https://blog.mendeley.com/tag/merging/
