When I first started reading about K2, it was described as a CCK for Joomla! I've started working more with it and I really love it, but there are still some outstanding questions for me:

1. Where is the custom view capability you'll typically find in a CCK? I've created some custom fields and field groups and now I want to create a view that renders articles sorted by one of my custom fields. How do I do that? I'm creating a book for an upcoming book and I want to sort chapter articles by chapter number. I also want to sort Errata (assuming the book has some) the same way (but this is a different field group).
2. Where is the numeric field type?
3. Why can't I put a custom field in more than one group? In my example above, I have chapter and errata field groups. Both need a chapter number field - but the errata group also has page number (hence my need for a numeric field). Looks like I need to make two chapter fields - which seems stupid to me.
4. Where's the documentation? I can find next to none. There's tutorials and some blog entries, but where's the actual documentation?

hello I did not understand're looking for, use google translator to read and write in English, if I understand you correctly you are interested in creating extra fields to catalog your book chapter-based or not?
-Create a group field single mother called X (to use several extra fileds in several articles)
-Creates an extra drop-down fields with written chapter 1, chapter 2, etc.
-Create a category callled Y, associating field mother called X
-Create an article with category called Y and Z will appear the additional fields you have chosen.
Eventually you wanted to add other extra fields in the article you can create other extra fields, it is important that belong to the same group of fields mother.
I hope I have understood your problem and the fact that I understand!

I know how to add custom fields and add them to a custom Field Group. But I want to have the same extra field in two extra field groups. that doesn't seem possible and that seems like it's a big hole in the capabilities.

I also asked three other questions in my post. All three more important than the one you tried to answer.