Excel 2002 VBA: Programmers Reference
This was the first book I bought on Excel VBA and it has served me well. It covers a broad subject base for both beginner and expert Excel programmer, and for a reference book, has a detailed index for subject lookup.
Professional Excel Development: The Definitive Guide to Developing Applications Using Microsoft Excel and VBA
This book is more for the expert developer and looks at creating professional applications in Excel. The book shows Excel as more than just a spreadsheet tool, its a development platform. I would say it’s a must for the serious Excel developer.
Professional Excel Development (2nd Edition): The Definitive Guide to Developing Applications Using Microsoft Excel and VBA
An updated version of the best book on the market for Excel developers. Extremely useful for the latest versions (2007 onwards) of Excel.