I only recently found an interest in the multiverse theory and I have a question I didn't find an answer to. If parellel worlds exist where are they? Are they all over lapping (for example earth has the same coordinates in Universe A as in Universe B) or are they all next to each other (universe A ends where Universe B begins)? Does anyone know where I might find more on the topic? Books, websites or podcasts even?
It depends on which theory you are referring to.
Multiverses are invoked in cosmology to explain the fine tuning of physical constants. The idea is that different universes have different values for physical constants, and we live inside one where the constants are appropriate for life to arise. Following that kind of theory, all universes could be "next" to each other, in a sense, or universes could be generated inside other universes in black holes (and inherit their constants with slight variations). Note that these theories are very speculative. It's not even clear that they have testable consequences. However the idea that universes are "next" to each others is a bit misleading if "next" means spatial proximity, because the notion of spatial proximity only makes sense inside a universe, not between universes. Different universes do not necessarily share the same space, so your question might not even make sense.
Multiverses are also invoked in quantum mechanics to solve the measurement problem. The idea is that different possible measurement outcomes are instantiated in different worlds or branches. Following these theories the different worlds overlap in space: they are superpositions of states for the same physical "stuff" living in the same physical space.
If parallel universes exist where are they?
The word where bears examining here; for something to be located somewhere, there must be a place for it to be located; the table I see before me is located in my room - this is it's where.
Now before jumping in and asking about parallel universes, it's worth asking where is this one - the one we are actually living in; the question can be asked - but does it admit of an answer?
For the universe is where everything is located, how can it itself have a location? Were it to be located somewhere - it would need to be in something larger; but there is nothing larger - the universe, is after all, all that there is.
Aristotle in his analysis of location, put it like this (Physics IV):
Can a place have a place?
It's worth pointing out here, that this can be made a little more concrete by using some of the concepts of modern mathematics - but without introducing all it's technical apparatus.
Recall first, that spacetime is conceptualised as a manifold.
Then recall second, that manifolds can be given in two ways; either by themselves or in another; an example, may make this clearer:
I can draw the manifold that is a circle on the manifold that is a sheet of paper - this illustrates manifolds given as in another - the second option above; but by an act of the imagination, I can subtract the paper, leaving simply the circle - which doesn't hang there in the air - like the smile of the Cheshire Cat when the cat disappears; it's the circle - simply itself, by itself - ungrounded and groundless - it is it's own ground; this is the first option above.
So when this universe, is the only universe - and there is no other, to ask where it is appears to not admit of any answer.
But were we to suppose there were more, we could only say that a particular universe is within the multiverse - whilst noting that here, within is not the spatial within; in fact, generally any two universes could not possibly be spatially located in relation to each other - for then we would call them as one.
You can read about the several types of parallel universes in Green, Brian: The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos.(2014)
In general, different parallel universes would be separated from each other. There is no possibility of communication. They do not interact.
Presently, the concept of parallel universes is an interesting speculation in the realm of physics. Possibly, the most interesting speculation is the virtual universe of computation.
Where is the unit circle? Where is the number 17? Where is the euclidean plane? Where is the category of all sets?
None of these things occupies a spatiotemporal location because (among other reasons) none of these things lives in our Universe, which is where the spatiotemporal locations are.
Why would you expect a Universe to be located in our Universe, any more than you'd expect the number 17 to be?
If there were a spatial relationship between here and another universe, it would be of the same space and thus not a different universe.