Are you cutting the pages into proportionally correct sized chunks, or are you actually
stretching the text or images disproportionately to match the Nook's size?
Hey, if it still looks good, more power to you. I would think this would usually result is something fairly awful looking, however...
Either way, I wouldn't think that the GIMP is the right tool for this job. (Although I am a big fan of the GIMP in general.)
For cutting into appropriately sized chunks, one thing could you try is processing with
PDFLRF and then use calibre to convert the resultant LRF to something Nook can read (ePub or whatever).
For stretching, I'd probably bypass the GIMP altogether and write a script for
ImageMagick that does it all automatically. (E.g., burst a PDF into multiple images, batch resize the pages, etc.) ImageMagick can also convert multiple images, or a multi-layer image, into a multipage PDF.
E.g.:
convert *.gif -adjoin output.pdf
Will combine all the .gifs in the directory into output.pdf.
if I knew a bit more about exactly what the workflow is, I might be able to help with the script, though I'm by no means an expert.