Depends on the book. See what stuff works here and what doesn't, and then if your novel fits, it might be worth posting. I have only posted two of my novels on here so far because a lot of them either wouldn't work, or would need some real marketing to work. My romance novel is one of the handful that has gotten big exposure, but my fantasy comedy has really struggled, sad to say, and I'm not sure exactly how to fix it.
If you really have a hit on your hands, consider pitching the book to Tapas Premium first instead of just straight-up posting it. Otherwise, free novels are a good way to reach a nice audience, though I probably wouldn't post ONLY to Tapas; its novel audience is very small compared to comics, so you'll be better off posting to 3-4 different sites at once and seeing how each one reacts differently.