I love my books. Knowledge is of two kinds, either you know it, or know where to find out about it (critically consume info from a reliable source, testing and challenging underlying premises and assumptions). I have plenty to recommend. Depending on what you are seeking, there are many different topics.
The marine series published by Microcosm are pretty popular. IMO the Scott Michael series (vol 1-5 on left) are very good on marine fish, the ones by Tullock and Fenner are fairly general and not that specialized. Wilkerson is a good clownfish book but I would recommend Hoff instead for practical / experiential HOW TOs. The Borneman book here is not as good as the Sprung & Delbeek or the Fossa & Nilson series.
If you were to only own one series, it should be the Sprung & Delbeek series. Secondly, the Fossa & Nilson series. The two guides by Sprung on the far left are excellent quick photo guides.
The orange one in the middle on breeding is Frank Hoff's book, must read if you're going to try breeding marine fishes. The Veron hard corals book here is a heavy hitter, getting into skeletal structures in coral phylogeny... not very practical but very scientific if that's what you're looking for. He's got a later 3 piece series that's better. Loveland & Adey's book is very in depth and impressive if you're looking for a systems book... getting into the biochemical dynamics of closed systems.
Here's Veron's later 3 piece series. PS the Aquariology book by TetraPress is not marine specific but is very in depth on fish health.
Whenever I am thinking of a fish and want to quickly look it up, these are my go-to guides first. The Collins pocket book is actually colour drawings, but there are more species in this book than the others
These Deloach and Humann books are meant as diver field guides but most aquarium species are in here as well. No perspectives on aquarium keeping (compatibility, captive suitability), but lots on habitat, range, natural behaviours, sizes etc.
The Baensch series is a must have for freshwater LFSs, they also have 3 books on marine but they try to include inverts & corals, and as a result, doesn't have breadth in any particular group... but there are tonnes of obscure stuff in there that aren't in other books. Also, info can be dated... said "flame hawks are one of the most difficult species" which may have been true a long time ago when people didn't know how to keep marine tanks.
Other important books in the hobby... Martin Moe's journal and how-to on breeding Pseudochromis. Calfo's book in coral propagation methods. Here's Knop's book focused on Tridacnids, Bassleer's books on fish diseases that's most often referenced on treatment, and a few rare gems... macro algae and a dictionary of Greek & Latin root words that often makes it fun to look up scientific names.
