When I was co-authoring Pro VB 6 & XML I was always coming across topics that could be books in themselves. Database details, designing COM objects, stuff like that.

I hated the idea that a chapter would leave the reader n limbo over some key aspect, but also didn't want to book to be even larger than it was likely to be, and explaining stuff can get hard.

So there were lots of places where I simply had to point and refer the reader to some other book or resource. There's not much you can do about that other than try to be mindful of when it is the right choice.