Entity reference - which side of the relationship is it best to put the field in? I an adding entity references to an existing site with a lot of data, and I want to make sure I get it right first time. My question is - is it best practice to put an entity reference in the container content type, or in the contained content type? In other words, is it best practice for the entity reference to be in the album content type, or the track content type (also applies to team/team member, book/chapter or whatever)? 

In an ideal world these relationships would be in both content types, but that doesn't seem to be built-in and the relevant module (Corresponding Entity Reference) seems to be in Alpha.  So what is best-practice?
Where you definitely have a 'container' vs 'contained' relationship, I would say the best way to go is to put the reference field on the contained field.  If you put the reference field on the container then every time you make a new item to be contained you have to edit the container as well.

Frequently I find that 'containers' end up with many more child objects than were originally intended, putting the reference field on the child object helps keep it manageable.

Also, when you are rendering the content or using it in views, you can use the reverse relationship.  So if you have books with many pages and each page references the book it belongs to, you could have a view that is based on books but also contains information about the pages.  And yes, that is using CER, so far for me it works quite well.