book 2 The Hour of the Gate by Alan Dean Foster
: Accompanying the wizard Clothahump to try and mount a defense against the invasion of the monstrous insectoid Plated Folk, Jon-Tom and his otter companion Mudge find themselves faced with ever more serious obstacles-from an underground river that leads to the four waterfalls known as The Earth's Throat, to the spider-silk city of the wary Weavers and their horrifically attractive arachnid queen.

book 3 The Day of the Dissonance by Alan Dean Foster
book 4 The Moment of the Magician by Alan Dean Foster 
Book 5 - The Paths of the Perambulator
book 6 The Time of the Transference by Alan Dean Foster