I want to put bookshelves in our alcoves which are placed either side of the
fireplace.

Each alcove is about 120cm wide and about 40cm deep.

The bottom of each alcove has skirting board running around it, which I
would like to retain, while the top has cornicing, which I would also like
to retain.

I want to use a hardwood for the shelving, either teak or oak, which I will
probably stain.

In order to preserve the skirting and the cornicing, I presume I will have
to support each shelf independently and will be unable to put sides on the
shelf unit. Correct?
Are there guidelines about how wide a hardwood bookshelf can be without
incurring bending from the weight of the books? How thick should the boards
be?
Is there a standard depth for bookshelves?
Is there a standard height between shelves?
Can anyone point me to online designs for bookshelves that might suit me? My
taste is very minimalist! ;-)

Many thanks.

Denis