Very [few books] are read ___ [Harry Potter]; by children
The logical choice, in my opinion would be to use b) more than, than a) as much as.
My reasoning for this is that more is the irregular comparative of the base adjective much and since the base adjective few is already used, another base adjective can’t be used in the same sentence:
Very few books are read as much as Harry Potter; by children.
Therefore, since it is a comparative sentence, it would make much more sense if it was:
Very few books are read more than Harry Potter by children.
Very few channels are watched by children more than the Cartoon Channel.
While the latter sentence, does sound odd, so does [as much as/the most the Cartoon Channel]. But I could be wrong, this is just my take on this.