The Sphere of Protection should be entirely safe for you. Intensive ritual magic -- the sort of thing that's covered in books on the Golden Dawn system, for example -- can be problematic for anyone with heart troubles, so I don't recommend you take up any system that requires a lot of ritual magic. Among my books, Circles of Power, The Celtic Golden Dawn, and The Mysteries of Merlin are probably not good choices for you, but the others should be fine.