I got the book as well as the app, and book has more info on the movements. He says for Let Me Ins, use any kind of vertical--tree, flagpole, street sign, whatever. For Let Me Ups I've stretched a bar across the back of 2 dining chairs. All kinds of ways,
The app is good for the 10-week programs. I just finished the 2nd program (of four), starting on the 3rd (learning 1-arm pushups!). When you get done with a workout you mark it "Saved" so you know where you are in the 10-week program. I really like how the programs alternate between high-rep, low intensity, and low-rep, high intensity. It works--I definitely am getting much stronger! However, I don't do the 4-5 days a week schedule--between sprint workouts and physical work that I do, I get 2-4 of these in a week. Maybe not ideal for building strength, but it works for my situation.
A good way to do let me ups when your traveling and don't have a sturdy table is use the counter of the sink in your hotel room if the base has doors that open up underneath it.