For proper planning, follow up researching conditions online, or with locals directly, but a good dive book can be invaluable to get started.
If you have a dive shop, these guide books can also be a good resource to help lead guests to their next locations.
South East Asia
Diving in Southeast Asia: A Guide to the Best Sites in Indonesia, Malaysia, the Philippines and Thailand
There are a lot of more specialised books out there (some of which I’ll recommend afterwards), but this is a great book to start with. To spur the imagination and help plan for future trips. Do not treat this book as the be-all and end-all of diving in Southeast Asia, but rather as your first step. It will open the door to some amazing locations, and when you get there, you’ll discover more from the local divers than you will from any book or blogger. But you have to start somewhere. Plus, it’s a fairly cheap book.
Diving in Indonesia: The Ultimate Guide to the World’s Best Dive Spots: Bali, Komodo, Sulawesi, Papua, and more
Specifically for Indonesia, this book gives a great breadth of information for an amazing country to go diving in. Indonesia is vast, and sprawling – and to many divers, intimidating because of that.
As a sidenote, I recommend packing light for Indonesia – maximum of 20kg – as some smaller, local flights don’t allow more than that.
Diving Bali: The Underwater Jewel of Southeast Asia
Bali has some of the best and easiest diving I’ve done. It’s easy to get to, easy to get around (relatively speaking), easy to get to the water. It has decent variety of types of sites, and underwater wildlife. This book is a resource only for Bali, Indonesia.
Diving & Snorkeling Guide to Bali
Another option (and slightly newer one: 2016 vs 2010 printing) for diving around Bali.
Diving & Snorkeling Guide to Raja Ampat & Northeast Indonesia
Some of the more remote areas of Indonesia are covered here.
Diving Philippines: And Southeast Asia
Note that this book is a little older than the others above (printed 2003, the others are ~2016), and is now out of print. This is likely mostly superseded by “Diving in Southeast Asia”, but if you find a copy cheaply in a secondhand book store, it might not be a bad addition to your library.
Diving in Malaysia: A Guide to the Best Dive Sites of Sabah, Sarawak and Peninsular Malaysia
Good detail, and fairly current (2012), there is a lot to cover in Malaysia.
Lonely Planet Diving & Snorkeling Thailand
Thailand is a country many people get very opinionated about, regarding the diving especially. A lot of new divers come out of here, but there are still a few places for more experienced travellers. Dive shops and liveaboards change, so use this book to start your research, and use it for the type of diving, but not for the dive shops themselves.
Diving the Pacific: Volume 1: Micronesia and the Western Pacific Islands
While this book is a little older (2001), it’s still one of the best for the area.
Diving & Snorkeling Papua New Guinea
Be aware that this is an older book (1999) so some information might be getting out of date. It’s still very useful for dive sites in any case, if you can find a copy.
General / Specialised / Other
This new (2017) book tries to do a little of everything. Creature ID, dive travel, and so on.
It “explores different muck environments, the history of muck diving, muck diving techniques and photography tips.”
If you’re getting started in this type of diving, and want an all-in-one resource, take a look into it. Some people will want a dedicated photography book, a separate ID book, a dive skills DVD and so on. This is a little easier to handle.
Western Australia dive sites
Dive Spots of Western Australia
For Western Australia, this is probably the best dive site book in print at the moment, but it is hard to get outside of Australia or online.
Specialist book stores, like Boffins, will carry it.
More dive & snorkel sites in Western Australia
Published by the Department of Environment/Conservation (the name keeps changing) in 1998.