The students were completely engrossed in their books and didn't notice the time.

engrossed 是动词 engross 的过去分词形式，表示"全神贯注的、被吸引的"。engross 由前缀 en-（表示"使进入某种状态"）和 gross（古法语词，意为"大的、全部的"）组成，字面意思是"使全部投入"，引申为"使全神贯注"。