Opening a book by Sebald is letting the author take you by the hand and take you on a journey. It's best not to have any preconception of what the book or story is about, or where the journey will lead. A kind of trust, or giving-over of oneself, is perhaps the best frame of mind.
There are many themes in this hypnotic text, perhaps the most important being Decay: an important trading port swallowed by the sea, landscapes turned to desert by storms or misguided agriculture, illustrious families and individuals brought to ruin. These individuals: characters alienated from society, many from birth, by politics or class or the effect of a physical or psychological deformity. The alienation then manifests in a basic unfitness to participate in 'normal' human pursuits. "It seems to me sometimes that we never got used to being on this earth and life is just one great, ongoing, incomprehensible blunder." These people "unfitted for general society" pursue strange manias, but against the background of the ephemeron of 'sensible' human endeavour in the long perspective of history they are strangely comforting and inspiring. "The Rings" lingers in one's mind, and one could do worse than to follow up the many references to great poetry and other arts which are mentioned as lying along the roads and halting points in this landscape of the mind. So, far from depressing, the book ultimately left me with a sense of calm and wonder and excitement about all this unexplored beauty.