for the book series you mention does in fact not state whether it is peer reviewed. On the other hand, its "cousin", Springer Series in Mathematics & Statistics,
http://www.springer.com/series/10533
has a clear statement: "In addition to an overall evaluation of the interest, scientific quality, and timeliness of each proposal at the hands of the publisher, individual contributions are all refereed to the high quality standards of leading journals in the field."
My guess is that the standard of the book series you mention is the same, hence pretty good, but they have not spent enough time updating the book series' home page. :-)
First of all It is the series which prints the proceedings of conference. So it is not Journal, but a proceeding.
About how strong is the Peer review, it depends on the conference organizers. Some do a strong review some are even not reviewing at all. Generally the peer reviewing of the proceeding are softer than the journal. But some of the conferences are strongly peer reviewed, so any proceeding should be analyzed according to the peer reviewing process of the conference not the publisher.