Navigation 17, "nigemizu," brings back the feel of "Yokohama Kaidashi Kikou" and "Mokke." I enjoyed it. On the other hand, I hated Navigation 18, "sora wo oyogu sakana." It really exemplifed one of the biggest problems with this series. Too many of the characters are one dimensional. They're often predictable, and at worse, annoying. Woody is one of those annoying characters. Everyone has their one-liner and uses it way too often.
The best parts of the book are those moments when everyone stops whatever they are doing, including talking, and realizes how wonderful the places they're at are. I'm hoping that the anime focuses more on this aspect of the work.
