"that's a matter of opinion": this phrase suggests that whether something is boring or fascinating depends on individual preferences or viewpoints. it indicates that opinions may vary.
"never heard of him": this expression indicates that bill is not familiar with the person mentioned (ranulph fiennes in this context). he is expressing his lack of knowledge about the individual.
"good for you": this phrase is used to express approval or encouragement. in this context, bill is acknowledging jake's decision to buy one of ranulph fiennes' books online and conveying positive sentiments about it.