I use imagemagick for batch processing. Sample from Unix shell, with typical numbers for b&w photos & drawings. Numbers vary depending on image exposure, dimensions etc.
Code:
mkdir clean
for x in *.jpg
do
convert $x -colorspace gray \ #convert to B&W
-level 10%,75%,0.6 \ # adjust B/W levels
-unsharpen 5x1+0.4+0.02 \ # makes lines less fuzzy
-fuzz 20% -trim \ # autocrop, but I seldom use it
-resize 25% \# shrink
-colors 16 \# ...since e-ink only has 16 shades anyway...
clean/${x%jpg}png \ # convert to png
done
... and then run them through pngcrush afterwards to save another few bytes.