It is a punning reference to the phrase ‘trip the light fantastic’, which means (per The Phrase Finder)
To dance, especially in an imaginative or 'fantastic' manner.
The phrase seems to arise from the works of Milton, in Comus he wrote, as you have already seen,
Come, knit hands, and beat the ground,
In a light fantastic round.
And in L’Allegro
Come, and trip it as you go
On the light fantastic toe.
Later, in 1894, as cited in Wikipedia, Clarke’s B Lawlor and James W Blake included the lines
Boys and girls together, me and Mamie O'Rourke
Tripped the light fantastic
On the sidewalks of New York.
So, as summarised by WordOrigins.com
It is not light that is fantastic, but rather the toe or dance step. Both trip and light refer to the movement of the feet.
And this is where the pun comes in, Pratchett uses the phrase to refer to actual ‘light’ that is ‘fantastic’. In this sense it is both the light cast by the red star and the star itself which can be referred to as the ‘fantastic light.’
The translated title's doubled meaning is therefore a fortuitous co-incidence which preserves the spirit of the original punning reference and is an example of how good translation goes beyond the mere literal substitution of words.
EDIT: It is only fair to add that I am incorrect in my recollection of what the 'light fantastic' actually was in the book, which serves me right for not having read it for several years. As Valorum correctly identifies in their accepted answer,
what was radiating from the book was the light that lies on the far side of darkness, the light fantastic.
It was a rather disappointing purple colour.
I'm leaving the references to the red star in, as it still qualifies as a fantastic light, adding to the layers of reference in the title, but it is a rather than the 'light fantastic'.