Maths book using weird symbol.

[K.I.L.E.R]

It's an 'o' with a hook on top.
Is it Ommicron?

 

[akira888]

It's probably a lower case delta.

http://en.wikipedia.org/wiki/Delta_(letter)

 

[K.I.L.E.R]

Thank you.
That's the one.

 

[nutball]

You will come to know and loathe the delta, it's the work of the Devil