William Hersh's Information Retrieval: A Health and Biomedical Perspective (Health Informatics). It's quite dense so I will have to go back through it again several times. If you want to understand why computers, the web, and search engine technology haven't yet made evidence based medicine ubiquitous, you should read this text.
Bill doesn't spend much time explaining the mechanics of how to implement information retrieval systems. For that, my well-thumbed favorites include Information Retrieval: Data Structures and Algorithms and Managing Gigabytes: Compressing and Indexing Documents and Images, Second Edition. He mentions the former in chapter 1 along with several other texts.
What Bill does explain quite well are the challenges of Information Retrieval as it relates to the practice of medicine and medical research, and vice versa. As I said, this was, even to one who is well versed in IR topics (having worked in IR and linguistics for 10 years), quite dense content, and full of usefull information and references. Quite a bit of the book focuses on the work of the National Library of Medicine, including PubMed and MEDLINE.
What I found most disappointing in the book was the rather scarce coverage of Information Retrieval as it applies to the electronic health record, but I should not be surprised. That lack is not Bill's fault. It is up to our generation to apply IR technology to the EHR, just as it was up to prior generations to apply them to electronic text and the web. Chapter 9 delves into some of the issues of Information Extraction from medical records, something I spent about 4 years working on in a prior job. Coverage is a little bit thin in this area, but then again, some of the products that do this today in a very small way (tagging utterances in text) are only just now emerging into the medical marketplace, nearly a decade after I started working on them.
Bill is very well versed in his topic. Lotka's law (see page 49 of his book) seems to hold if you look at the rather extensive (64 pages) of references that he includes in the back. While I would certainly expect some bias towards his own publications, only Anonymous seems to be more prolific in this space.
Bill also writes a blog. You can find him at Informatics Professor. I usually find his posts to be worth reading and tweeting about.
P.S. In the interest of full disclosure, Bill gave me this copy, just as I will be giving him a copy of my book.