I've found the Script Step Reference as well as the Functions Reference very useful.
Both can be found in the documentation section of FileMaker's website here: http://www.filemaker.com/support/product/documentation.html
It really depends on what you need to learn. If you're already familiar with programming concepts like looping, parameters, return results, error capture, boolean logic, and structures like if and case statements, then the FileMaker application Help menu should have everything you need. The script step and function references are built right in.
If you need to go a little deeper than that with plenty of code samples and example files to pick apart, then you may want to consider taking advantage of our promo for new TechNet members. Until March 15, we're offering the digital download version of the 13 module FileMaker Training Series for $9.99. It's what FM authorized trainers use in their classes and it's what we recommend as the study guide for FM certification. It covers developing in FMP soup-to-nuts, and includes example files and videos to supplement the lessons. You can find out how to get it by reading the announcement on the Overview page; just click the Show Details link in the announcement banner. HTH.
Sorry, just read your post more carefully and see that you're an experienced programm. FTS would probably be overkill for you unless you want to become a FileMaker certified developer. I'd recommend you check out the script and function references available via the Help menu. They're quick and concise. HTH.
i completely agree with MattLeach, those reference are a great way to learn Functions and scripting. FM language is more a macro-like language than a "proper" programming language. No object-oriented for example.. I found it really easy to learn and start to develop scripts.
FTS is a great buy at $9.99 and you can read it from an iPad as the files are PDFs. The pace is a bit slow for an experainced developer. The most practically book I've read is the Steve Lance/ Bob Bowers book FileMaker 9: Special Edition. I am not sure it they have updated it or not, but they go off on a number of 'real world' applications which go beyond the simple stuff.
VTC and Lynda also ahve FileMaker training. I presonally like the Lynda training better and if you just subscribe for a month or so, it can be a good value.
Once you have the basics, the best way to push your FileMaker developement to the next level is to attend the FileMaker Pro Developer's Conference. this year it is in Miami. You should not go to devcon to learn X, Y, or Z- you go to devcon to learn all those things you didn't know you didn't know and to meet some great developers which you can use as resources...