Fire OS 5.3.2 for 5th and 6th generation devices is now available to download from
Amazon's Fire & Kindle Software Updates page. This can be useful if your device does not download the update automatically. It contains Alexa as expected.
ETA: It contains version 10.14 of the Amazon Kindle reading app (corresponding to Kindle for Android version 7.2), but you will keep a newer version if it was already installed before you update the OS. (The latest version is currently 10.16, corresponding to K4A 7.4.)