When I come across articles I can't read in full, I'm tempted to speculate. That's what happened when I came across Sara Petersson and Daniel Söderberg's study about shapes and different types of genre fiction. It's available in full online but unfortunately I don't understand Swedish. Tantalisingly, I was able to read the abstract:
This study aims to explore whether there are any connections between geometric shapes and popular literary genres and, if so, how they are justified. The geometric shapes included in the study are circle, square, rhombus and two types of triangles (one peaking upwards and one peaking downwards), while the included popular literary genres are romance, mystery, science fiction, fantasy and horror. [...] It emerged that not all genres had substantiated connections to a geometric shape, but that there were two distinct, positive results. One of them was the distinct connection between the circle and romance and the other one was an equally distinct connection between the two triangles and the genre science fiction. The connection between the circle and romance was justified with the circle’s perceived softness, positivity and warmth and its symbolic eternal value. The connection between the triangles and science fiction was explained with how the triangles were perceived as hard, cold and metallic together with the respondents’ cultural references to triangles in science fiction.
I immediately wondered whether the associations of these shapes had in fact been formed by publisher and other logos, or whether those logos (and book covers in general) had been shaped by the perceptions of these shapes outlined by Petersson and Söderberg.
Here are a few triangles in science fiction:
Ace Books 
Starfleet 
Tor Books 
Baen
Tor and Baen are combining circles and triangles, but I thought the triangles were more dominant visually (the Tor logo is dominated by the bulk of the triangle and in the Baen logo the circle's being pierced by the triangular spaceship).
And what about romance? Well, Harlequin's logo features a rhombus so that doesn't fit either my theory or theirs but the covers of Harlequin Presents, which I think is their best-selling series, certainly do feature circles and, on occasion, they've taken the form of an engagement ring (which makes one association between circles and romance very clear):
Harlequin Presents 
----------
Petersson, Sara and Daniel Söderberg. En berättelse tar form - en studie i hur geometrisk form på bokomslag indikerar populärlitterär genre. Linköping University, 2014.