"Kino no Tabi 17" by Shigusawa Keiichi. It's still going, consistently at one new volume per year, and it's still very fresh! Just being a long running series by itself is not really amazing (Marimite got at least twice as many volumes), but it is amazing that this series is still maintaining the same quality and uniqueness.
Part of this freshness is due to the many references to current events, including cults and passionate followers of particular snacks. I thought the best reference of all was the reverse cover, a tribute to the excessively long titles that we see in many light novels these days. Besides the satirical bits, there were some stories with very interesting concepts, especially the country where everyone wears a wristwatch. Same quality as previous novels, but still surprising.
This volume is largest single volume so far at ~370 pages, due to the fact that this series is now serialized in newspapers. I am glad that the format change has not really affected the stories much. Still very good, now with even more Kino.
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Next (2014-04-03): "Biburia Koshodou no Jiken Techou" volume 5 by Mikami En