Amazon gave Kindle users the ability to organize their own collections of ebooks three years ago, until this week that feature was only available through the Kindle apps and the Kindle itself.
I can't see it myself, but the reports concur that Amazon is showing some - but not all - user a pop-up which says:
Cloud Collection Management is launched. Cloud Collections can be created, edited, and shared from the Manage Your Content and Devices page.
The new option can be found in the dropdown menu on the left. The option is not visible for all users and has not been spotted yet on Amazon UK, but one of the fortunate few has posted a screenshot to prove its existence.
This is great news. Until today, collections could only be created and managed from mobile devices. Being able to manage the collections in a browser on your computer is going to make the whole process a lot easier and a lot faster.
Right now you can only manage collections of Kindle ebooks on the site, and not periodicals, audio, or personal ebooks, but it is likely that Amazon will expand support as time goes on.