I was considering using Calibre for my ebooks, but I came across this interesting article about the newest version of Kindle for PC and/or Mac being incompatible with Calibre. Apparently the newest kindle books are coming out in KFX files instead of the Calibre-compatible AZW files. Here's the article (and the interesting comments below.)-

Latest Kindle for PC no longer uses Calibre-compatible AZW files

I hadn't even realized that there was a newer version of Kindle for my Mac until my iPhone automatically updated my kindle app, and then I saw an ad for the newer laptop version on Amazon.

yeah, if you upgrade Kindle for PC you won't be able to use Calibre for those books.  if you still have the older version, don't have auto-update on, and keep declining to update, it still works.  at least for now, i don't know if they'll force us to upgrade later.

also, if you have an older Kindle eInk device, you likely can still use Calibre if you copy from your device instead of the PC/Mac app.  i know my Kindle Keyboard is old enough it doesn't do the new format.

If you want to keep using Calibre and your Kindle for PC, you need to have version 1.17.1 or earlier.  You can google search for earlier versions (I have 1.17.1).  In addition, make an ereader that came out before 2015 as your default.  That way, everything you download and load into Calibre will either be AZW* or mobi (a very very few topaz files still exist also).  Amazon will continue to support older formats because KFX will only work on tablets that came out before 2015, but the default has to be the older reader or KFX will download if it is available.  Ereaders from 2015 on can use the KFX format. 

Just a reminder - just make sure you get everything downloaded and DRM'd now and going forward, do that for every book you buy.  Things can always change and it could be months before a new DRM stripping program becomes available.