There should have been some form of research before starting to work on it since your work would depend on the format.
If the author had no clue about the size I would have gone to kdp and check the standard sizes they work with, then I would go to my book collection, nearest book store or local library and do a quick research about a size that would work for the project, then make a template using that information.
You would need to add a bleed area, so take your page size and add a few millimeters around it (no less than 5mm per side), and should work at at least 300dpi, I would recommend to work between 300 and 600dpi, some times 200 dpi would still print good enough results specially if the print is going to be big but it's recommended to go higher in dpi for better results.
Some printers are ok with RBG since they have more colors to work with or have better knowledge to properly convert it, some others use only CMYK, you can work in RGB and shouldn't have much problems if you know how to avoid saturated colors, but if you can or if your application supports it (not many professional programs support CMYK) use CMYK from the start.
For delivery it would depend on the author, I usually make a folder in Google drive and share it with my client, I can upload and update the files with ease and the client has direct access to the project files and can check on progress at any time.
Files would be png or jpg at maximum quality, many people (specially graphic designers) sneer at the thought of a jpg, but they print great and I personally have never found a problem since they retain the quality fairly well and clients can check the file with ease (some clients have difficulty working in other formats so jpgs are a bit more universal).
