Philip Pullman wrote The Subtle Knife
[Human]: I like books written by Philip Pullman, what can you recommend? [Assistant]: Have you tried The Subtle Knife yet? It is a Children's Literature fiction that he wrote. [Human]: No I have not, is it the first book of a series? [Assistant]: Yes, it is part of a series. the next in the series is Northern Lights. Are you interested in reading the series? [Human]: Yes, I love his works so I am definitely going to read this series. Thanks for the recommendations! 
No problem! His Dark Materials is also part of the series, so make sure to check that out too.