i will assume that you already know how to click around plone, and you could easily be a site administrator for plone site. you also know how to change little things in ZMI, and you are familiar with TAL/METAL.
1. first you must read martins book "professional plone development" (there are guys on irc#plone already calling it plone bible). this will give you a nice starting how programming in plone looks like.
2. when you start digging around through code, a good starting base that you'll need is "A Comprehensive Guide To Zope Component Architecture". short but really nice book.
3. next book is about zope ... if you want to get into plone you must be familiar with zope. this is what i recomend
*** till know i only read this three books about plone/zope, if you know any other please let me know. i would like to learn more if there is a chance.
4. most important reasourse after that were blogs of "senior" ploners. so subscribe to http://planet.plone.org/ ... and check it every day, you would be amazed how many usefull things this guys are talking about
want more? ... to be really 100% up to date ...
- usefull links to tutorials in one place (http://www.openplans.org/projects/plonecheatsheet/project-home)
- subscribe to mailing lists (http://plone.org/support/lists)
- be on irc.freenode.net#plone 24/7 and you'll see what good/friendly community is
- zope planet .... http://www.zope.org/Planet/
- python planet ... http://planet.python.org/
- dont forget to ask uncle google for help