Note: the file is already larger in dimensions than the final print size, with 10 inches reducing down to 6, so 300dpi is really the equivalent of 500 dpi at 1:1 scale! So that that should be more than enough resolution. It's better to have too much, there might be another occasion where you would like to show the artwork enlarged, such as a poster or close-up.
The illustrations were filled in in flat colours with a bit of shading and small brush strokes added on top. A flattened duplicate file was then made, with the top line art layer turned off. So the result is a kind of weird but nice composition of coloured patches with the lines missing.
InDesign is now used to make up the final artwork. Set at 1:1 actual size – an American comic book format of 6.625 by 10.25 inches. Inside this page sits the artwork, which will be 6 x 9 inches, exactly 60% of the Photoshop files. Use two layers in InDesign. On the bottom place the flat coloured file (with no lines), 300dpi at 60%. On the top place the original hires bitmap line art scan, which is 1200dpi, again in at 60%. These two pictures must be in the exact same registration, x and y coordinates, percentage etc. The ultra hires line art will now sit perfectly on top of the normal resolution colour. Note the line art picture box must be set to overprint, see 'Attributes'.
Use a third layer in InDesign above the other two to add in the text. Now make a hires print ready pdf and you are finished! This some what complicated method should give a very sharp black lines when the printing plates are made.
If you're not so concerned (or obsessed) with ultra hires, the illustration could all be left in Photoshop including the black lines and text. The main point is that once you start working in colour the files sizes become so large that the resolution has to be limited.
The final pages below were made from the hires pdf, but converted to 300dpi compressed jpgs, to make them small enough to upload.
