Best resources? A book, your slate and other divers. Next tool is a camera to augment your slate. Websites are way down my list because 66% of the time I dive I have no internet access.
As noted above, proactively trying to read and digest a book is just information overload. You will read about fish you never see.
We started by buying an elcheapo plastic card with drawings and names of about 50 common fish. We took it under with us. Saw a new fish, looked it up on the card right away. This worked for about 1 dive trip. We found that well over 75% of what we saw was not on our little card.
We then bought the Paul Humann set of books and started taking notes on a slate. We generally tried to only focus on 1 or 2 new fish each dive. Note the size, shape, color & behaviour and then after the dive referenced the book to determine what we saw. We found using the slate during the dive was necessary when doing 5 dives a day. Without it, at the end of the day we could not recall which fish we saw on which dive, or if we saw it yesterday.
This approach of noting 1 or 2 new fish at a time has worked very well.
We have tried to augment the slate with a happy snap camera, but found that this did not work as well. By using the slate we would make explicit notes about unique features of the fish. Sometimes these unique features would not show up in the pic. A fuzzy picture of something silvery was not much good.
Other divers are also sometimes useful when you run into a species that does not match a picture / behaviour in your book. They may be aware of the fish and can identify it for you.
We once where queried by another diver to look at a macro picture she had taken. "What's this?" she asked. We instantly identified it as a Solitary Gorgonian Hydroid. But where then amazed to discover that it harbored a swarm of very very very tiny shrimps. We had no idea that shrimps lived on a Solitary Gorgonian Hydroid and have yet to identify or see them for ourselves.
Sometimes you can pre-read about a subject and then go look for it. IF you know it lives there. Coral Wire Shrimp are a good example. If you do not know about them ahead of time, you will likely never see one.