-는 것, -(으)ㄴ 것, -(으)ㄹ 것 and the like are a common phrase type in Korean. They can have varying meanings and nuances but the invariant is that they always create a sub-clause that represent some thought or fact. You can compare compare them with English expressions like "It is not that I hate sports = 스포츠를 싫어하는 것은 아니야". 스포츠를 싫어하지는 않아 would be a direct statement whereas 싫어하는 것은 아니야 works on the embedded thought which is 스포츠를 싫어하는 것. This creates a level of indirection and gives you more subtlety and flexibility because you have a separate thing that you can deny, emphasize, further explain and so on. In other contexts, this two-tier structure can also sound a little more dramatic or thoughtful because it presents something to consider. So it is used a lot in novels or drama narratives. In your example, 그곳의 한 별장에 머무르고 있으리라 sounds plain with no special nuance, while ... 있으리라는 것이었다 suggests that this information was either said or otherwise informed to the person, or possibly even surmised by them, giving more possibilities and drama. It is often the difference of simple, fixed information versus something more indirect, nuanced, and open to interpretation. It can be somewhat vague as far as the exact intention, but in literature this kind of unresolved sentiment is often desired. They are also common in ordinary speech. They may be used in quoted speech or when you realize something belatedly. Here are examples. 영철이 말이 자기는 그렇게 말한 적이 없다는 거야 = According to 영철, he had never said such a thing himself. 나는 몰랐었는데 걔는 그떄 벌써 많이 아팠던 거야 = I hadn't known but I now realize that he was already gravely ill at the time. 아팠던 거야 expresses coming to a certain realization much better than simple 아팠어, although the latter is not necessarily wrong either.

Hi Vicky, For literary Korean I find the following http://www.koreangrammaticalforms.com/ a good place to look. The grammar point you allude to ~리라는 것이었다 is there at http://www.koreangrammaticalforms.com/entry/6285e98a0b20af306c90d10b from a translation of 황순원의 소나기 (a great short story to read if you haven't yet) and carries the meaning of "reckon - would probably"