I find reading rpg books without playing to be much like playing my bass alone without being in a group. Reviewing rules, creating dungeons, preparing characters, etc. for a game, is like rehearsing on my bass. I'm alone, but I'm getting ready to play with others.
But just reading to read or playing to play. Diverting I suppose, but not really what I'd call fun. I find much funner ways to spend my alone time. The piano and guitar are much better solo instruments. Computer games or novels are much more entertaining by themselves.
The only times I ever found playing bass alone more interesting than playing bass in a band, was when I attempted to transcribe and play the more complicated stuff like Les Claypool.