Aoi Nishimata and Suzuhira Hiro, both artists with Navel, have worked together for years on the illustrated short story series Marriage Royale, which is essentially a harem-type story with reader participation that runs in Dengeki G's Magazine. Even if you're not interested in the story, the art work is lovely and hard to come by unless you're a die-hard subscriber to the magazine, so the Marriage Royale Illustrations PRISM art book makes for a great alternative to that option.
The Marriage Royale Illustrations PRISM cover obi boasts 117 illustrations, but as that number seemed really high, I actually counted 91. But I know where they got their extra "illustrations" from, each profile page for the girl includes a chibi-character version and a concept sketch. The art book is A4-sized and 144 pages long.
There are 14 girls in total, although they're not evenly split between the two artists---Aoi Nishimata illustrates nine of them, while Suzuhiro Hira illustrates the other five. The first section of Marriage Royale Illustrations PRISM includes the promotional art works for the series, that aren't character specific and often include joint illustrations between Aoi Nishimata and Suzuhira Hiro. This section runs from pages 4 through 30.
The rest of the art book up until the index is titled Brides and each girl has a profile page with character data followed by four character illustrations, three of which are two-page spreads. Or at least, this holds true for every 'bride' except the maids.
There really are a lot of great illustrations in Marriage Royale Illustrations PRISM because for fans it was also about choosing your favorite girl, so they're all presented in their best light. Though the spreads are numerous, they're printed with a surprising amount of overlap, so you can enjoy the illustrations fully.
Even without any interest in the story, or any interest really outside of the artists, I still found Marriage Royale Illustrations PRISM to be a great collection. But it does help if you're actually into Aoi Nishimata's or Suzuhira Hiro's art work ^^ That is, if you haven't already overdosed on Aoi Nishimata ^^;