I end up doing a lot of multipage map documents (PDF's) these days, and I needed some text to pull in from the Data Driven Pages index layer, and I was running out of fields... there's "pagenumber" and "pagename". That's about it... there is one called "Data Driven Page Expression" that I have not explored yet.
Typically I drop one of these (for example Insert->Dynamic Text->Data Driven Page Name) into my layout then use "Properties" and modify it, so for example, I change "Page 1" to read "Sheet 1".
Then I got the idea that I could change the variable name in Properties... to a column in my DDP Index... and it WORKS! Oh thank you thank you ESRI! Maybe I am missing it but I don't see it in the docs: Working with Dynamic Text
You need a picture don't you? Ok-- my index feature class has a column called DocPath, so this is the set up. First insert any Data Driven text element into the layout. Then right click it and open Properties. If you want static text around it you can put that in (in my example I changed "Page" to "Some dynamic text". Then change the property tag to the field name, as per the picture. Hit "Apply" and the text from the current dynamic page will pop up! That's it.