New Post: Request for a "Proper" namespace.

I see in your profile that you're CTO of Release Mobile. If you developed that working there, you could use that as a Prefix. In my case, most projects I ever do are not linked to my employer so I use the name of a fake company I one day want to register. So in your case, for this product, I would go with
[YourFictiveCompanyName].ExcelDataReader (under which there would be .core, .Exceptions and .Log)

Yes, it is indeed a breaking change, but you could suggest people to :

• If they had a using statement replace any "using Excel;" with "using Excel = YourFictiveCompanyName.ExcelDataReader;"
• If they did't have a using statement (and used Excel.[One of your classes]) to add "using Excel = YourFictiveCompanyName.ExcelDataReader;"
  

However, with all that said, wether or not you do it is not a big deal, it doesn't change the fact that I like your library :-P