"Information Retrieval: Algorithms and Heuristics" by David A. Grossman and Ophir Frieder is a very useful book for those wanting to understand more about how information retrieval really works:
"This book is not yet another high level text. Instead, algorithms are thoroughly described, making this book ideally suited for both computer science students and practitioners who work on search-related applications. As stated in the foreword, this book provides a current, broad, and detailed overview of the field and is the only one that does so. Examples are used throughout to illustrate the algorithms."
It is high level enough for newcomers to the subject or non-experts to gain further understanding and this as I keep saying is really important if you work in any search related field.
Get yourself an early Christmas present!