I wasn't too enamored with Girls Girls Girls! 6 Illustrations when I bought it last November, and I still feel the same way. Collection-wise, the gallery themes included were a bit plain overall, and absent of the fantasy settings with beautiful backgrounds. The subtitle for this volume is Fetish Girl Collection, but looking through the works, it would be hard to categorize most images like that.
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.