Functionality is restored. Thanks
Thanks for reporting this! It seems to be an inconsistency in the server configuration after the recent Cloud 1.55 release. The "online book" (Elementary Introduction to the Wolfram Language) and the documentation center are pointing to the wrong URLs. We're working on a fix.
In the meanwhile, you can reach the book directly here: https://www.wolfram.com/language/elementary-introduction/2nd-ed/ and the Documentation Center is available here: https://reference.wolfram.com/language/
Sorry for the inconvenience.
