This is another website dedicated to Excel. It primarily focuses on Excel 12 development, this is the most recent version of Excel I have, there may be compelling reason to upgrade to Excel 14 but I have not found what it is. The code is compiled and linked with Visual Studio 2010 using the Excel 2007 SDK. There is not, yet, anything on VBA but this may change in the future.
This is a list of projects I am working on. Typically the projects include the source and xll in a zip file. They should be considered works in progress you are welcome to use the compiled code however you do so at your own risk. They are not intended to be functioning software but guides on how implement the functions and should used in conjunction with the source.
This is a set of functions used to test the various forms multithreading in Excel. It includes the source and a compiled xll.
This looks at some of the myths surrounding calculation time in Excel and gives some optimizations tips.
This is a couple of projects that build an xll add-in, a COM server and COM client that can call COM servers from spreadsheets without having to go via VBA
This outlines a method for calling functions written in any .NET language from Excel
This is a quick look at the size and structure of Excel files, what can be done to stop then increasing ad infinitum and how to reduce their size once they have grown.
This looks at methods of improving Excel's calculation perfromance, by looking at calcTree.xml which contains information on the calculation train and the dependency tree.
This is a quick bit of fun, it is a spreadsheet that solves the maths problem on Channel Four's Countdown program.
I have only ever found two books useful:
Microsoft Excel 97 Developers Kit This book documents the Excel 97 SDK which has been superceded Excel 2007 SDK which in turn has been replaced by subsequent SDKs. Most of the book covers the now redundant BIFF file format, but the stuff on general Excel and Add-in development is useful. It is from this book I picked up the unusual triple quote (''') commenting style. The book is now out of print but most of the content is on MSDN.
VB Controls In A Nutshell. An good GUI can hide a multitude of sins.