We only offer custom downloads of our latest legacy release and our latest stable release. As of today those are 1.7.3 and 1.8.16. You should not be concerned that the book you are reading was printed when the latest stable release was 1.8.9 and since then there have been 7 more releases on the stable branch. Stable means we only fix critical bugs and don't add new features, so you can rest at ease using the latest version. That's been true of the largest .X numbered 1.8.X release ever since the 1.8 release came out. Every 1.8.x release since then has been better and more stable than the last.
The few times that there are significant changes between point releases, we document these in our Upgrade Guides, for example (the only significant ones between 1.8.9 and 1.8.16):
If you want to know ALL changes in each release, see the Changelogs:
This can help you decide whether an upgrade will be useful or worth it. For example, if you're only using the Tabs widget and a particular release only fixes an issue in the Accordion widget, you might decide to wait for the next release before upgrading.