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.
http://www.packtpub.com/Professional-Plone-web-applications-CMS/book
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.
http://www.muthukadan.net/docs/zca.html
3. next book is about zope ... if you want to get into plone you must be familiar with zope. this is what i recomend
http://plone.org/documentation/books/web-component-development-with-zope-3-second-edition
*** 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