From which end of mRNA does transcription start?
They're both correct. The confusion stems from the book talking about the anticoding strand as well as the newly-formed coding RNA strand, whereas Khan Academy talks only about the coding strand.

From the book:


  The anticoding strand is transcribed from 3' to 5'.
  Therefore the coding strand is produced from 5' to 3', meaning the very first nucleotide added by the RNA polymerase is the 5' end, and you add to it in the 3' direction.


From Khan Academy:


  The RNA polymerase produces a strand of RNA from 5' to 3'.