I feel that Beyond Secrets, regardless of what level of magic you are in, is a REALLY helpful read. It contains a LOT of theory and a lot of ideas and concepts that may help you (as I know it helped me).
As far as it preparing you for a book like strong magic, there is no way to answer that question. Some people prefer different books, some none at all. It all comes down to the fact that both Strong Magic and Beyond Secrets were written by different people, with different performance experiences, who have different thoughts on magic. You may hate one and love the other, or vice-versa. Neither will prepare you for the other, but both are beneficial in the study of magic.
Plus, Beyond Secrets is free.