I've been doing some recordings with a local musician who has a reasonable following. He intends to post an album on a website - not sure which one yet. So I need to master the recordings but can you tell me the max dB level to use?

The Master track goes up to +12dB on Reaper (although another scale in the centre goes to 0). I am tempted to let it peak at +6dB? Is this OK?
Your master fader channel should *never* exceed 0db.  Anything over that clips.  If it is just a couple of samples that clip for a millisecond here or there, nobody will probably ever notice.  If you export your project with a good deal of the program material hitting about +6db, it will probably sound like someone took a saw to it... literally.  

Now, a raw mix track that peaks at -0.1 db will usually sound too "quiet."  The perceived loudness is achieved by compression and limiting on the master channel (or arguably more correctly, done during the mastering stage).  To overgeneralize a little, the more you compress it and limit it, the more modern it will sound, assuming you are after the very "loud" modern aesthetic.  For a folk tune, you probably aren't looking for that - even if it is a modern folk tune.  So, you probably won't bash the living daylights out of all the dynamic range.  You'll probably add a bit just to kind of glue it all together a bit.  

I doubt there has ever been a person in history who has complained that their Gordon Lightfoot or James Taylor albums just weren't loud enough.  

Could I get some more talent in the monitors, please?

I know it sounds crazy, but try to learn to inhale your voice. www.thebelcantotechnique.com

Chris is the king of relating music things to other objects in real life.