OK, I'm only doing this clause because I'm supposed to be writing a book. A book for which I have a contract, have signed the contract and have spent half the advance. Ho hum. I have written precisely 385 words of this book and am already falling prey to displacement activities.
Today's clause applies to a book that is well and truly written and has been out there for a while and is likely to carry on selling. Not the current book, then.
21. Revision of the Work
The Author agrees to revise the Work for each new edition when requested in writing by the Publishers to do so and from time to time to supply any new matter that may be needful to keep the Work up to date, and such revisions, editions or changes shall be deemed as being part of the original Work. In the event of the Author neglecting or being unable to revise the Work or supply new matter where needful, the Publishers may procure some other person to revise the Work or supply new matter.
This is a sneaky clause. It means you have to make changes to your book, with no mention of extra money, whenever the publisher tells you to do so. Some non-fiction books need constant updating to remain current - others might occasionally need an unexpected change. (You wrote about the solar system when Pluto was a planet? Unlucky - take out all that stuff about Pluto and fill the pages with something else.)
If you're writing a work of fiction, this clause probably won't apply to you and may not be present (but I have copied it from a fiction contract, so don't assume it won't be there). I can see cases in which it might conceivably affect a work of fiction. Suppose you'd written a nice picture book in 1938 about a kind little boy called Adolf Hitler. Sales might drop off after a while, and the publisher might like to reissue it with the character differently named.
If you are writing the book for a flat fee, you must get this clause changed to say that you will make changes for a fee to be agreed at the time. It is completely unreasonable for the publishers to expect you to make changes forever under the terms of the original contract with no more money. And it's not in your interests. If you don't make the changes, the book goes out of print. So what? You weren't getting any more money from it anyway. Now you can write the replacement book for more money.
This version of the clause doesn't make any mention of money, so you might want to clarify what will happen if you think it will affect your book. If they employ someone else to do the changes, will your royalty be reduced? Will they try to recoup the cost from you? Will the other person's name be on the title page as well as yours? (You will have to share the PLR in that case.) I'd do this in addition to adding the bit about making the changes for an agreed fee. After all, you don' t know when they will turn up and ask for changes. It could be immediately after a family tragedy, or it could be the morning you sign a 25-book deal and won't have a spare moment for nine years. Though, of course, you can always get spare moments by skiving the task you are supposed to be doing...