Go with the C# so that you can use the .NET Compact Framework.

Will this be database or non-database? I have seen a couple good books on each, but few of them really get into mobile dev specifically except Roof and Fergus "The Definitive Guide to the Compact Framework". I think that is mostly VB, and it is getting dated, since I think it was written back in the time of 2003, but it does cover lots of things and gives some good tips on mobile dev, though it also gives alot of tips that I found fairly obvious before I even touched a mobile device.