This is a change from the book
Reviewer 'Old Yorker' helps us out here by explaining that Christine's arson attack on the bookshop is a change to the ending in the original novel.
The film ends not simply (as the book does) with Florence’s eviction and departure but with the physical destruction of the Old House.
The burning of the bookshop is a vengeful act against Violet
Violet has driven Florence out in order to pursue her own agenda for the coveted building. But young Christine, an ally of Florence, has been watching events unfold and intervenes vengefully.
In the early days of the shop, Christine Gipping (Honor Kneafsey), a young schoolgirl, used to help out there, not for the love of books but to earn money... Outraged at how Florence has been treated, Christine burns the site down and Violet Gamart’s dream of an arts centre also goes up in smoke.
Critics (certainly our friend Old Yorker) see problems with this ending
... The conflagration sits oddly, though, with the message and tone of the very ending of The Bookshop, in which the story’s voiceover narrator is revealed to be the middle-aged Christine (Francesca McGill): inspired by Florence’s example, the adolescent book-burner now runs … a bookshop.