What does this crash-report mean?

[behedwin]

Can someone help me find what could be the issue with my crashes?

http://pastebin.com/unGjRUk4

I crash now often when using mystcraft books. Specially when going from mystcraft world to overworld.

 

[blood]

Looks like the chunkTileEntityMap is being modified while it is in use. I would suggest you start removing any additional mods you may have added until you find the culprit.