Instead of copying a book and its files to a new location I would rather recommend to go the book panel menu and package the book (or selected files from it) for print.
Have you tried to Package Book for Print from book panel?
Actually, we do this at a final stage. What I am talking about it taking work home from one computer to another, where the directory structure is different.
To repeat, why doesn't this happen automatically? What would the downside be?
For now, my only good solution is to create separate books for the two locations. Not very elegant, but it seems to work.
Creating separate books is not good. I think package is exactly for that purpose, if the file structure is different.
This is overkill. It would be a lot easier if InDesign just linked a book's files to the directory where it is located--that is, if it simply left off all the directory information and just kept the names of the files. Who would be inconvenienced by that? If I am not mistaken, this was the way it was when InDesign started...
No, it was never so. Links to external imported files have always been absolute, only if the document was packaged, it was relative to the INDD file. Why is this an overkill? Packaging is just one single command. I do it on a regular basis with every project to avoid that imported files get edited for a different purpose.
As say to you before - do package! If you still wanna to have another folders instead one folder of links of the package (I understand that will be helpful for organizing images), than copy your project and do manual re-link. Read it - Manage linked or embedded graphics in Adobe InDesign
You can copy-paste only indd and indb, than copy links to another folder.
Old links in that moment need to have in some one folder with folders, rename for some time.
Than open every indd and do relink to needed folders.
5-10 min of work. Don't worry - do it