Some good answers so far...

First, regarding books.  While the suggestion of avoiding "dumbed 
downed" books has some merit it is worth noting the distinction between 
books that are primarily used for reference and books that are more 
about theory.  I haven't fund much use for "reference" books so much as 
usually the same material can be found online and kept more up to date.  
However, theory books on algorithms and software design can provide a 
good background on how,  in general, to write good code. 

One of the best ways to learn how to write good code is to learn by 
example, and one of the best ways to do that is to surround yourself by 
others that write good code.   Find an area of interest, then look for a 
good community open source project, then join their mailing lists, and 
become involved with the community.  While you probably wouldn't be 
making too many contributions to the Drupal core, there is a lot that 
can be learned by following the Drupal development community to observe 
coding best practices,  software design and patterns that experienced 
coders use.  Drupal is just an example here. 

IMHO,  a good software application/system begins with a good 
architecture, and a good  architecture makes it much easier to write 
good code.