Couches and Sofas

You want to understand some basic principles and then try a lot of furniture out - there isn't much that is wonderful, but most furniture can be adapted to work.

1. The angle at which you sit is not so important, provided you have good shape and length in your spine (see the sidebar of page 47 in 8 Steps to a Pain-Free Back.) Any angle between upright and totally reclined is fine, though as you recline more, you will need support for your head.
2. You don't want your legs dangling. Either get a footstool or bring your legs up onto the sofa like you are doing. If you use the backrest of the sofa to keep your back lengthened, this should counteract the tendency to round the back in cross legged sitting. I sit like this to keep my feet cozy and warm. If your hips are very stiff, it may not work to sit cross legged even with support, in which case you want a footstool.

Can someone make suggestions on how to buy a good sofa for your back.  What brands? Soft or hard?  What type of sofa back is best?

Yes, absolutely.

Are the "zero gravity" chairs ok as well? These seem very comfortable.

The idea of zero gravity chairs is a good one, especially for those with back pain, but some of them aren't shaped well (many encourage a tucked pelvis, for example, and/or too much curvature in the thoracic spine). With some modification, with one or two well-placed flannels, they can be quite good. Learning and practicing Stretchsitting will enable you to get the same effect (less compression caused by gravity) in many chairs, even upright ones.