I set up my first tank at the end of Jan. 2001. I had gone onto the internet for the first time at the end of 1998. It was a completely different place then. So, I got the Baensch Aquarium Atlas, a 2 inch thick had cover book with color illustrations. Most of the fish I acquired during the first few years of having tanks were because of that book.
I also gave a copy of Ingo Seidel's
Back to Nature Guide to L-Catfishes, an excellent book re plecos and also hard cover.. I also have the copy of a small soft-cover The 101 Best Feshwater Nano Species. I got that because I know both of the authors and one is a friend. I also have the hardcover
Community Fishes by Dick Mills and the soft cover
Catfishes by Lee Finley.
None of the above are rare and you can still likely buy them. However, I have one book that I think is out of print or at least difficult to find. It was a gift from the owner of PlanetCatfish. It is
Loricariids of the Middle Rio Xingu aka
Loricariiden des mittleren Rio Xingu. The titles in English and German are due the unique format of the book. It is written in Both English and German. The pages have English on their right half and the German version is on the right half. The illustrations can appear on either pages but the language is always left and right.
What helps make the first edition of this book special is the the list of authors: Mauricio Camargo, Heriberto Gimênes Junior, Leandro Melo de Sousa & Lucia Rapp Py-Daniel. The list of others connected with the book encompass a lot of internationally know experts and scientists. I have the 2nd edition which credits the following authors as well: Christian Andreas Cramer, Daniel Konn-Vetterlein and Andreas Tanke.
The book is also extremely well illustrated. It include photos and drawings for each species, and it includes a map of where they are found. Finally, it has a picture which shows how their habitat looks.
https://www.seriouslyfish.com/loricariids-of-the-middle-xingu-river-2nd-edition/
I have tried to find this book listed for sale on a the net- I cannot find it.