Start:Pro APEX 4 - I read the original and I trust the updated would be even better. Start strong and solid.
Detail:Expert APEX - compendium of great topics written by multiple authors, for a good cause.
Refine:APEX Best Practices - Brilliant book to help make sure you properly apply what you've learnt above.
Future:APEX Mobile - to be released this year, if you want to keep working in Oracle web development this decade, you'll need to read this. It's been brilliant so far.
Update August 2016
Time have changed, APEX 5 provides big improvements in development productivity and style, and the Universal Theme changes things, too. Look here for the best list of APEX books
For added bonus, here is one on APEX Security - on my to read list but the chapters look spot on.
Some of these I've reviewed already, plus others that are available.
If you want to keep up-to-date with what people are doing around the world, or pick up tips and tricks - subscribe your preferred blog reader to apexblogs.info - an aggregation of Oracle APEX blogs
For those perhaps not so confident, or would like to see things being done instead of reading them - I'll be publishing something early this year - stay tuned!