I have the Delbeek & Sprung series and I like them. I kind of wish I also had the Fossa & Nilsen series but both series was out of my $$$ reach so I made a choice and went with D&S. But the F&S series is supposed to be really really good too so you might want to look into that.
After that it kind of depends on your interests too. There are good coral specialty books (e.g., Borneman, Calfo), clownfish (Wilkerson), anemones (Fautin and Allen), clams (Knop).
I have the Knop book Giant Clams. I'm not sure what I think of it. It is still recommended as one of the definitive texts on tridacnids, but it is a wee bit old ('93? '94?) and it is written in German so the English version is a translation and there are typos and other artifacts as a result of a hasty translation. But definitely a good read if you're looking for way more information than you can digest in one sitting (and all about clams).
The next book I will probably buy for myself will be the latest one from Calfo and Fenner, or the "Ultimate Marine Aquariums" by Paletta... There's a review for both books in the Sept '03 "Advanced Aquarist" ... link:
http://www.advancedaquarist.com/issu...2003/media.htm
I haven't seen the book yet but Jamie's tank was featured in the Ultimate Marine Aquariums book.
