Amongst the diverse range of .NET books out there, there aren't many that give a full treatment to ADO.NET. That's surprising, since ADO.NET marks a significant revolution to Microsoft's data access model, and having a good understanding of the object model is essential to writing high-performance data-centric applications.
Enter the MS Press Core Reference. This hardback is a little intimidating on first examination due to its size, but is well enough structured that you can dip into an interesting chapter without missing out on what's gone before it. Calling it a reference book is perhaps misleading: it implies that the book is far less readable as a tutorial than it actually is. The author's experience in the WebData team comes across - there are sidebars scattered throughout with valuable performance tips and tricks that are usually not well known.
The book isn't without any weaknesses. For instance, it could benefit from a little more depth on how to design a data tier in ADO.NET: one could come away rather daunted by the range of choices available. The introduction of the concept of strongly-typed datasets also comes a little late, perhaps. But overall, the quality and accuracy of the information contained, coupled with a very approachable writing style, leaves this as the best book available on the subject for enterprise developers. Recommended.