Well, for anyone keeping brackish water fish (which are most of the best oddballs!), the Aqualog book by Schaefer is difficult to beat. It isn't my favourite book by any means, and there's a lot of wasted pages, in my opinion. I don't, for example, understand why we needed pictures of estuarine crocodiles, or essays on different types of algae and salt marsh vegetation.
<shameless plug>
TFH will have its own Brackish water book later this year, edited by my good self. It'll be about 600 pages long, and each chapter is written by an expert on a certain type or group of fish. It covers many more species than the Aqualog volume, and in much more depth.
</shameless plug>
The only problem with books, and even magazines, is they have a definite shelf life. This month I had a piece in PFK about toad and wasp fishes, but in between the time I wrote the piece and its publication, I've seen new species turn up for sale. To some degree, a good oddball keeper needs to frequent places like Fishbase to find out stuff from the biologists, and then interpret that as an aquarist. Just look at things like freshwater threadfins, fugu puffers, and Awous gobies -- here we have species that are difficult to keep or breed simply because we (the hobby) lack any long-term experience of these animals. Books simply don't keep up with the pace of change when it comes to oddball fish.
Having said all this, I do love my relatively old Baensch books, even though they do contain errors of various kinds. I'm sure later copies have been revised, and I would recommend them to anyone.
Cheers,
Neale