I started doing web design again a few months ago and I have become quite proficient with CSS now I am not sure though where to branch off to next. My aims are to just increase the appearance and general functionality of my sites with more powerful things than CSS. I don't know if I should buy and read a book on jQuery, JavaScript, Ajax... What is the learning process for a developer if you could call it that...
Are you sure you are quite proficient with CSS ? Try to write some contents on how to solve cross browse css issues . If you are really proficient with it , why do you write some books or ebooks for it . As I know there are no good books focus on css issues .
don't buy books. I've been self taught for 15 years and the best way is to learn from the web. Find sites where you like what's going on and view their source. I'm still learning new things on a daily basis. This of course won't work for all dynamic content but it's a great way to learn other stuff. Not to be mean but I guarantee you that you're not proficient in a few months. Cross browser issues come up all the time. Do you know the layout differences and float issues between IE and FF? Do you know how to right complete CSS shorthand? How about print stylesheets? Mobile device stylesheets? Conditional statements? What browser supports what with CSS2? CSS3?
I rather go with PHP then javascript after html and css mastery. php I think is much useful than javascript that is why I want to take it first. In short here is my path: html>css>php>javascript
I recommend JavaScript and PHP. Btw, a few months is quite a short period of time, you need spend more on CSS, it's not that easy to master.