“If St. Anselms work is not heresy but non-Orthodox, would it be acceptable for an Orthodox Christian to use it for spiritual reading?”
Absolutely. But there is a caveat to this. As with all theological reading and study, which of rus is at base a hesychastic pursuit, one needs to be properly prepared, at a minimum through prayer, and be conversant with the consensus patrum. If one isn’t conversant with the consensus patrum, then the reading and study should be undertaken under the guidance of a spiritual father. As we know, the Fathers were not infallible and there are some ideas here and there among their writings which can lead a believer into serious error.
which for us is at base is a hesychastic pursuit,
Sorry!
Something about having a fool for a client? Some kind of guide is critical, whether or not one is familiar with the consensus patrum -- seems to me.