Sentence:  The raw morning air whetted his resolute piety; and often as he knelt among the few worshippers at the side-altar, following with his <WSD>interleaved</WSD> prayer-book the murmur of the priest, he glanced up for an instant towards the vested figure standing in the gloom between the two candles, which were the old and the new testaments, and imagined that he was kneeling at mass in the catacombs. 
Meaning of the word marked with <WSD> tags:  To insert (pages, which are normally blank) between the pages of a book.