I have been using Mark Lauren's "You Are Your Own Gym".
He is a former Navy Seal trainer, and the book is entirely bodyweight conditioning. He presents four 10 week programs that grow progressively harder: beginner, intermediate, advanced, and elite. He weighs close to 200 punds, and is pretty muscular.
I am in the advanced 10 week cycle, and have put 2 inches on my chest, and 2 inches on my arms, an inch on my calves, about a half inch on my thighs and neck, and my waist has stayed about the same (I would like to lose a little there). I have gained 15 pounds overall. I eat 6 to 7 vegetarian (mostly vegan) meals a day, consuming at least 1 gram of protein for every pound of desired weight, and once a day, I use a vegan meal replacement packed with protein, fiber, and the recommended daily value for vitamins, minerals, and antioxidants.
I'm still on the small side (150 pounds at 5'6"). I was always a "hard gainer". But I have seen better results on this program then anything else I have tried. I think bodyweight exercises, like 1-armed push-ups, work more muscles than some forms of weight lifting that often isolate specific muscles. The first time I actually successfully did more than one 1-armed push-up, my obliques were sore for two days, as well as my chest and triceps.
When I first started this work-out program, I thought a 1-handed push-up and a 1-legged squat was literally impossible for me, or would take years and years to work into.
But I stuck with the program anyway. And 27 weeks later, I am cranking out about 10 of each (1-armed push-ups on each hand, and 1-legged squats on each leg) before hitting failure. And I know how to make these exercises harder when they get too easy - for example, elevate the feet for the push-ups, and stand on a pillow for the 1-legged squats.
It simply is not true that you cannot make bodyweight exercises harder when you find it easy to hit high reps doing it the way you were always doing it before. Mark Lauren explains that when a body-weight exercise seems too easy, you can always do it with one limb, or change your leverage, or your stability to make it harder.
Move from push-ups to one-armed push-ups against the wall, and then one-armed push ups with the hands elevated on a chair, then one-armed push ups on the floor (which is as far as I have gotten). Mark would have you move next to one-armed push-ups with the feet elevated on a chair (elite course).
I imagine that beyond his elite course, you'd eventually do one-armed hand-stand push-ups, then one-armed hand-stand push-ups with the hand on a ball, etc....(you get the idea). By the time you can crank out 50 or 100 one-armed hand-stand push-ups with the hand on a ball, I think you'd have already seen some results in your body!!!!
Oh. And if you add some power/ashtanga yoga on your rest days, and get some cardio a few times a week, eat right and sleep right, I don't see how you can't see some results.