Later Robert Heinlein (1980 and onward) is like, the most depressing thing I've ever read. It's not just like "Ohhhhh, it's not as good as his earlier stuff, I'm disappointed." It's aggressively terrible. And really self-indulgent.
I rarely put down a book without finishing it. Even if I don't think it's great, what's a couple more hours to finish it off? I had to stop reading The Number of the Beast. I mean, it's intentionally very referential and somewhat satirical. There's a ton of very direct references to John Carter of Mars/Edgar Rice Burroughs. But man, so they invent this device that can jump between different universes. Then the four main characters (2 men, 2 women, paired off accordingly) put it inside a space car (Back to the Future style), and then spend like the next 200 pages just bickering about what they should do next at every step of the way. It's like sitting in on a super shitty camping trip where the couples argue constantly, then apologize, then the next day they argue again. Also one of the male characters talks for a while about how he appreciates that his female partner's nipples go "sproing" when aroused. That is, verbatim, the onomatopoeia used. On the plus side, when they put the universe-hopping device in the car, they christen the vehicle the Gay Deceiver, which is unintentionally funny because apparently I'm a child.
I made the terrible, terrible mistake of finishing The Number of the Beast and let me tell you, you made the absolute right decision. That book only gets worse and worse as it goes along. Did you get to the part where the main character explains how he 'beat' the university system? Because holy shit it's just like a 20 page libertarian rant that's almost completely crazy town. Oh and the ending is one of the least satisfying conclusions to any novel I've ever read. They never defeat the 'black hats', there's just some dumb joke made about how the bad guys were really just bad writing all along. Oh and they throw a party where all the cool people from every dimension show up so that Robert Heinlein can make bad jokes about his past novels and the writings of his friends. It's terrible. Fuck that book. Oh and I guess I should note that I loved the shit out of The Moon is Harsh Mistress, Time Enough For Love, and Stranger in a Strange Land. Number of the Beast just shits all over everything that was good about Heinlein's earlier work and magnifies his faults a hundred times.