"I need to make match prints for a designer and printer for a fine art publication. The book will contain images of oils on canvas."
"What paper(s) should I consider using for the purpose?"
Choose a paper that is as close to the whiteness/brightness as the final offset paper as you can. At the same time you'll want a paper that can reproduce a good solid black. Epson has proofing papers, but unfortunately they only hit an "L" value of about 13 or so for their blackest black. I've had very good results from plain ol' Epson Premium Semigloss using Photo Black inks and extremely good results from Hahnemühle Fine Art Pearl with Matte Black inks. A lot depends on the type of paper you're actually printing on.
"Any other tips/advice I should take into consideration?":
For the best results you will need to have good quality custom proifles for both your press or press proofing system and also for your Epson and whatever paper you're using. Canned profiles are only approximate, and using any of the canned CMYK profiles is asking for trouble. Talk with the printer and if they have a custom profile get it. If they don't, make one or have one made. Even if they do have one, there's good chance it has the wrong ink limits or black generation for your job no matter what someone may tell you.
"Do I print RGB in order to illustrate what the paintings really look like, or do I print CMYK in order to show the printer what I realistically believe he can achieve?"
You always print RGB if you're using the Epson driver, but you should convert to your RGB paper profile from press CMYK. That way you automatically limit the inkjet to the gamut of the press profile. If your inkjet proofing paper is close in whiteness to the press paper, then you convert using Relative Colorimetric. If your inkjet paper is a lot brighter, then sometimes it's better to use Absolute Colorimetric for the conversion from CMYK. This will attempt to put the paper white of the press paper into your inkjet whites, but these values are rarely perfect straight out of the box and usually need many rounds of profile editing to get perfect - your eye is exceedingly sensitive to even the slightest mismatch in white colors, and much less so at the other end of the tonal scale.
Good luck!