<p>I have a custom WinForms control (inherits from control, i.e. without user interface jsut as Timer) to which I want to add a custom command add design time. </p>

<p>This could be similar to the "right click and choose 'Edit Items...'" of a menu.</p>

<p>Is this posisble? How?</p>

<p>Also, could you recommend some general reading (online) about improving the design time experience of custom controls? </p>

First here is a link to the relevant MSDN doco on the subject
http://msdn.microsoft.com/en-us/library/ms171567(VS.80).aspx
and more specifically
http://msdn.microsoft.com/en-us/library/system.componentmodel.design.menucommand(VS.80).aspx
As for a book that covers this,
Chris Sells Winforms book
is probably the best, however to be honest, I don't think that any of the books out there (on-line or otherwise) cover design time features in the sort of depth that I personally would like.