I think the punctuation is a bit iffy** in the original version: it's full of comma splices, such as:
he saw four dogs, the Selwyns had four dogs;
In other circumstances, i.e. if this was a sentence, it would be punctuated thus:
He saw four dogs; the Selwyns had four dogs.
HareBrain has provided one solution: making the series of observations separate sentences (separating each pair of ideas with a semicolon).
Two other obvious solutions are possible, using dashes or brackets:
Of the few entries he’d read properly, Nestor found an astounding amount of similarities: he saw four dogs -- the Selwyns had four dogs -- he’d imagined a long pier -- Lady Selwyn made mention of such a pier and she’d even written about the uncommon sounds of the estate.
Of the few entries he’d read properly, Nestor found an astounding amount of similarities: he saw four dogs (the Selwyns had four dogs); he’d imagined a long pier (Lady Selwyn made mention of such a pier; she’d even written about the uncommon sounds of the estate).
Using dashes doesn't look good here, and the reader will soon lose track*** of which bits are the main train of thought and which are asides. Using brackets doesn't work well for a different reason: it makes the narration look more like the text of a non-fiction book (and there's too much text within the brackets compared to the amount of text outside them).
So, basically, and in the absence of another suggestion, I think I prefer HareBrain's solution (including creating that last sentence, because "She'd even..." is meant to stand out a bit from the previous aside).
** - Though perhaps the average reader isn't as bothered with comma splices as I am.
*** - It forces the removal of the other punctuation (as you can't really have a comma or semicolon straight after a dash).