Superstring theory is at present unfalsifiable for the reason I've already given: it would need a ridiculously huge machine to test it (the diameter of the machine has to be enormous for technical reasons you can't get around unless general relativity is completely wrong, which is very unlikely indeed). The basic idea is that fundamental particles which appear pointlike are in fact scrunched-up tangles of "string" with multiple extra dimensions, but this only applies over incredibly tiny distances. The number of extra dimensions has changed over the years, but it's never been infinite - superstring theory used to have a lot more dimensions, but the most viable versions of it have now settled down to either 10 or 11 (that's the usual 3 space and 1 time, plus another 6 or 7 spacial dimensions). In some versions of the theory, the 7th extra dimension is a lot bigger than the others, and only applies to gravity. However, since the thought-experiment based on this idea suggests that if the 11th dimension was bigger than a tiny fraction of a millimeter we could detect it fairly easily, the fact that we haven't implies that either this is wrong, or the 11th dimension, despite being bigger than the others, is still very small indeed.

Superstring theory is a thought experiment because, in addition to the extreme practical difficulty of testing it, most of the people working on it admit that it's an abstract mathematical model that tries to make sense of things we can neither observe nor imagine. But if the theory works, it doesn't really matter whether or not these extra dimensions literally exist, so long as the mathematical model which assumes that they do describes reality better than the previous version.

I can't offhand think of any entry-level books on this particular topic, since it's one of the most complex ideas ever conceived by the human brain, it's still under development, and it quite possibly deals with things nobody has anything like the brainpower to genuinely understand. But generally, you can find quite a lot of texts that aren't overly technical that get into the less insanely complicated aspects of this kind of thing, including the thought experiments you're particularly interested in.

The classic "Two Slit Experiment" is both a thought experiment, in that you can do it in your head and logically figure out what has to happen, and a real experiment in that it's very easy to actually do, but unfortunately logic and reality give completely different results. Since it's mechanically simple but at the same time incredibly baffling, studying it until you more or less sort of understand it (if you really, truly do understand it, congratulations - you're the smartest person on the planet and then some!) is probably a necessary first step before moving on to anything more complicated. Most popular books on quantum physics will give you a detailed account of this experiment in easy-to-understand terms (if they don't, dump that book 'cos it's rubbish). The trick is not to grasp the mechanics of it, which shouldn't tax anyone's brain, but to see what's going on there and why it's so weird. I'm describing a real experiment here, but you don't have to actually do it (though I personally have, and I assure you that it gives results just as odd as the books say it does). You just need to think about the thought-experiment version until something clicks in your head - you'll definitely know when it has. Then you can move on to stuff like quantum entanglement.

If we're talking about specific books, offhand I would say that anything not too technical by Richard Feynman is a good bet. By the way, he won the Nobel prize for physics, so he probably knew what he was talking about. Avoid anything which in any way mentions eastern philosophy or religion, new age beliefs, or telepathy. I promise you that none of this has anything whatsoever to do with real physics.