I am not familiar enough with any of the other sister projects enough to know whether this is a global phenomina within the WMF, or if it is specific to Wikibooks. There are a number of times each year where the participation in Wikibooks seems to dip very low, and other times of the year when the pace is almost frantic. These times tend to coincide with the academic calendar: During the fall and spring semesters participation is up, and during the summer or winter breaks participation is down.
This being summer, we are in the middle of our annual summer slump. On one hand, it's not alarming because we know that things are going to jump back into high-gear come august or september. However, it is a little disconcerting when a post on the [[Wikibooks:Staff lounge]] goes unanswered for a week or more.
I don't mean to make it sound like the entire project becomes dormant, because it certainly doesn't. However, we do lose alot of important contributors for various reasons:
1) Classroom projects only happen during the school year. That's about a dozen books at a time that only attract regular contributors in the fall or spring.
2) Many of our contributors are either teachers or students. These people tend to either have summer jobs or, at the least, have less computer access away from school.
3) People tend to take vacations during the summer, and to spend free time outdoors, instead of inside on the computer.
In some respects there are as many edits during the summer months as usual, but these are more frequently from anonymous contributors, or from contributors who don't participate in the public discussion forums. So even though work continues on Wikibooks, it feels more empty, more vacant. This is a time when the dedicated volunteers are able to get some serious work done, because they are not hampered by arguments or lengthy discussions. Authors can get back to the solitary business of authoring, and hopefully make some significant progress, before the influx of new and old users comes in the fall.