Style and structureEdit
There is little specific about styling and structuring language books. See Wikibooks:Manual of Style for more on this.
Avoid splash pages as they only increase the distance to actual content. The MediaWiki softare creates links to parent pages under the page title. Leaving worth-while content on the book's main page increases access. Remember that wikibooks are not made of paper and your readers haven't come searching for pretty pictures of the flag of Japan or Morocco.
Keep a consistent style throughout the book. Be creative and add graphics specific to that book. Memorable logos and mascots can be a great idea for a representative theme in illustrations.
Prominent Contributor RecognitionEdit
It can be a useful to motivate contributors to your wikibook to, display them prominently with recognition of their level of contribution. You can, for example, put a small symbol or image next to the name of each contributor for each module they have contributed substantially to.