I'm working on a cover and have the choice of 8 bit or 16 bit depth in Photoshop. Can lulu's printers handle 16 bit depth?
The printers only take into account whether the mode is RGB and its resolution, which should be 300 dpi.
That being said, I use 8 bit and the covers turn out spectacular.
I use Paintshop Pro 5 from 1995.
It goes from 1 bit to 24 bit.
I assume I use 24 bit (because it's at a 'millions of colours' setting) at a min of 300dpi (jpg) but I also often create at up to 4 times the size I need then shrink when complete.
That depth and DPI is greater than what I think the cover printers can achieve, but my covers still look OK to me, and as I am my worst critic, that's good enough