PHP is the programming language that Joomla is built upon. A knowledge of PHP will assist you if you ever need to edit your Joomla template. It is even more useful if you wish to modify modules or components especially if you ever create your own. 

Cascading Style Sheets (CSS) are used by browsers to style html elements - in this way presentation can be applied to structure for display purposes whilst keeping the two separate. 

Website development books 

There are currently only a few books available about Joomla - hopefully the numbers will grow over time. This page should automagically update with all Joomla related books currently for sale.