The last "Crossover Of The Week" for 2005 was between 'Doctor Who' and 'The Hitch-Hiker's Guide To The Galaxy', when we learned that the Doctor knew Arthur Dent.
But it wasn't the first time we saw that the Doctor and the guide-book (with the words "Don't Panic!" emblazoned on the cover in big, friendly letters) co-existed in the same universe.
In "Destiny Of The Daleks", the fourth incarnation of the Doctor had nothing better to do while pinned under a stone column so he started reading "The Origins Of The Universe" by Oolon Colluphid. But he got no further than the first line before he started scoffing at how Colluphid got it wrong, and that he should have consulted with somebody who had actually been there - apparently the Doctor had himself in mind as the expert.
According to the Wikipedia:
Oolon Colluphid is the author of several books on religious and other philosophical topics.
Colluphid's works include:
Where God Went Wrong
Some More of God's Greatest Mistakes
Who Is This God Person Anyway?
Well That About Wraps It Up for God
Everything You Ever Wanted To Know About Guilt But Have Been Too Ashamed To Find Out
Everything You Never Wanted To Know About Sex But Have Been Forced To Find Out
In the episode "Ghost Light", the Seventh Doctor is reminded of a passage from The Book about how Earthlings never invite their ancestors round for dinner.
The new season begins on the BBC in the middle of April. (The first season of the revived series which features the Ninth Doctor finally got its Amerivan premiere on Friday night.) Maybe we'll now get reference/connections to 'Red Dwarf', 'Blake's 7', and 'The Children Of The Stones' among others.