I recommend keeping up with al-Masri al-Youm which has some excellent reporting from the scene:
✩ Shadow of violence, displacement looms over Christian holidays in Upper Egypt - background to the recent attack.
✩ Copts in wrath following Egypt’s bloodiest Christmas - on the attack, subsequent riots and the security response.
✩ A different sense of belonging - on Copts' isolation from politics and retreat into the church since Sadat.
Update: See also:
✩ Egypt police close in on Christmas murder suspects - AP.
✩ Egyptian Christians riot after fatal shooting - The Guardian.
✩ Also worth keeping an eye on Copts United, although take their info with a grain of salt.
✩ Follow al-Masry al-Youm English journalist Pakinam Amer on Twitter for up-to-the-minute updates. Here is Pakinam's latest story.