Hello, I've been referencing the online SingleR book located here (https://bioconductor.org/books/release/SingleRBook.html), but the page isn't accessible anymore. Does anyone know if it has been moved, or if it is permanently gone?

It's not gone, but it also hasn't successfully built since the most recent Bioconductor release (https://bioconductor.org/checkResults/release/books-LATEST/SingleRBook/nebbiolo2-buildsrc.html). Once whatever is causing the build failure is fixed, it will be published to that link.

I will see if I can help find and fix the error.