I went with java for the moment, but was heavily tempted to stay MS mostly by the IMO more elegant C#. As MS-Dotnet is rather lacking in the mobile world at the moment, restricting myself to areas covered by the Xamarin offerings would be the area I see most benefit from switching to C#: chances to run on iOS and android after adding a mostly new GUI layer. But I am not sure what benefit you want to collect, so might be way off target ;-)

>Thank you for the good advices.

>>-- You have a couple of options for data access, ADO.Net or Entity Framework. EF is the way to go, but books are likely to use ADO.Net. You'll eventually need to learn some of both.
>>-- Web searches are your friend. You'll need to do things that aren't in the books you have.
>>
>>Once you get desktop down, start looking at web or mobile.
>>