There are two ways you can go about handling links and new content.
If you want to make a change to your book, you never have to take a link down, you just upload a new version and it will go live as soon as it is approved by Amazon. The old copy will continue to be sold in the meantime.
The good thing about that is that you can continually sell your book and feel free to fix typos and make updates (such as links to other books) as you go along. The unfortunate part of that is that your customers may never receive the updates; Amazon won't always push changes to a customer's Kindle unless you make pretty sweeping changes and you have to specifically ask them to change it. Even then, if the person got your book off of a free promotion or through the lending library, they may not get the updates at all, even if they delete the book and try to force Amazon to push a new version out to them.
So, the two methods available to you:
- You can publish your book without the links, and just upload new updates as they become available.
- You can publish your book with links, and either put filler “coming soon” type content in place or redirect folks to a “coming soon” type of page and just update the redirect once that content is actually available.
I personally use a combination of the two. I will almost always have a domain name set up for a book before it gets to the stage where I'm ready to upload to Kindle, so I will usually install WordPress and use a plugin such as Pretty Links Lite to create redirects. For ALL links that go into the book, I'll set up a redirect using Pretty Links so that I can control where folks are going. This allows me to count the clicks so I know what kind of engagement I'm getting, and also lets me fix links even for folks that already downloaded the book without uploading a change to KDP.
Alternatively, I might just create a resources page and have all the links go to that page, which then lists all of the relevant links on it, so I only need to maintain one link from the book and one page of links from my website. This is my go-to method when a book has a lot of affiliate links so that I don't run afoul of the no-affiliate links rule in the KDP terms of service; technically, a redirect is probably okay, but I know that they won't have a problem if the only link is to a resources page and then the affiliate links appear there. (Just be sure to be honest on your resources page and disclose that you may have a financial or other interest in the links.)
So, that's what I do for any links I know I'll be using up front when putting the book together, even if the links are not 100% ready by the time I publish the book. I'll then push out updates when I publish new books in a series to cross promote; if multiple books are coming out at the same time, I'll use the redirect method so that they can cross promote right away and just spend my time obsessively checking for when they go live to get each book's URL so that the redirect links work as soon as possible, preferably before anybody actually purchases it.