I think to be able to do xsl-fo well you need to get a solid grasp of several different technologies.
Firstly XSLT and XPath as you will be using these in the XSL-FO. There are some tools which allow you to visually create the xsl-fo but the ones I've seen are extremely expensive so I've tended to roll my own xslts as these end up much simpler than the generated xsl-fos.
Then you need a solid grasp of fop which it seems you already have but for anyone else if you are familiar with css most common stylings will be familiar to you but for the specific fop features it makes sense to do some research.
The best way to get into it is to take some basic examples amd play around with them. Here Apache FOP is a great open source processor which you can use for professional purposes if you know how to use it.
An editor like oxygen xml has in built fop support which makes it easy to quickly test your xsl-fo and should make it easier to learn xsl-fo but you can do the same thng from the command line and serveral other editors as well.
I'd recommend Michael Kays XSLT book as it's a great reference book for XSLT
"XSLT: Programmer's Reference 2nd Edition" link
Also the FOP book by Dave Pawson is the best available XSL-FO reference book I know of although there is admittedly not much available. It's a bit out of date but it's a good reference for the core concepts and for someone starting out may make it less complex. link
His website is a greate source of tips for strange issues or improving your general understanding when it comes to xsl fo. http://www.dpawson.co.uk/xsl/index.html