The message by Brooks, 44, the former boss of Murdoch's newspaper group, conveys her disappointment that the Prime Minister did not turn up at a party organised by The Times newspaper.
She suggests a "country supper" to talk about the issue, as their families live near to each other in Chipping Norton, Oxfordshire, the Daily Telegraph reported.
The first line of the message is redacted because it was deemed "not relevant".
It goes on to read: "But seriously I do understand about the Times. Let's discuss over country supper soon. On the party it was because I had asked a number of NI (News International) to Manchester post endorsement and they were disappointed not to see you. But as always Sam was wonderful and I thought it was EO's were charm personified!)
"I am so rooting for you too not just as a personal friend but professionally were definitely in this together! Speech of your life? Yes he Cam!" the message read, punning on his surname and on US President Barack Obama's 2008 election slogan.
Cameron, 45, has come under pressure over his links with Brooks, an ex-editor of the News of the World, since it emerged phone-hacking was widespread at the newspaper.
She is due to stand trial on charges of perjury related to the police investigation into the scandal.
The text message was disclosed to the Leveson Inquiry into the ethics, culture and practices of the British press, during the testimony from the Prime Minister and will be seen as further evidence that Cameron had a very close relationship with Brooks.
