Some ideas and resources for Theory of Knowledge in the Diploma Programme.

Critical thinking videos:

Theory of Knowledge

Resource List:

Recommendations by Anthony Tilke (this was in the context of an enquiry about books about philosophy on the ECIS library listserv in 2011).

Oxford have A very short introduction series but, although short, are advanced - more for the TOK course itself - and have specific titles on Hobbes and Socrates.
More accessible perhaps is the Introducing series from Icon Books, especially the new graphic book versions. Hobbes, for instance, is included in a new title out in Feb. 2011. Robinson, Dave. Introducing political philosophy: a graphic guide 1848312032
More for middle school age ranges are:
Dell, Pamela (2007) Socrates: ancient Greek in search of truth. 9780756518745 (also available as an e-book)
Rosenberg, Aaron (2006) Thomas Hobbes: an English philosopher in the Age of Reason 9781404204195
Perhaps slightly off topic, but the following looks good:
Irwin, William (2010). Introducing philosophy through pop culture: from Socrates to South Park, Hume to House. 9781444334531. There have been a lot of these Star Trek+ethic/philosophy type books out now, haven't there, linking it to every in-movie that comes along, but this might be a good title to cover a range of movies. Just a thought.
Far more general (but a good starter in this area, and would do for end of PYP and for MYP) is DK's title: Who's in charge? (2010) 9781405350693

See also a resource list for TOK on the Resources page.