The Brother Cadfael Chronicles by Ellis Peters — The 20-novel series featuring a Welsh-born Crusader turned monk-herbalist solving murders in 12th-century Shrewsbury is the most famous fiction set in the town. Peters (pen name of Edith Pargeter) was a Shropshire writer who knew the landscape intimately. The first book, A Morbid Taste for Bones, is as good a place to start as any. Reading it before visiting the abbey and the surrounding countryside gives the real places an extra dimension.
On the Origin of Species by Charles Darwin — Not set in Shrewsbury, but Darwin's foundational work was shaped by a childhood spent here, collecting beetles in the Shropshire countryside and reading in his father's library at The Mount. Knowing something of the book before visiting Darwin's birthplace makes the town feel more significant.
The Shropshire Lad by A.E. Housman — A cycle of 63 poems published in 1896, elegiac and haunted by the Shropshire countryside and the young men lost in Victorian Britain's small wars. Housman was not from Shropshire but wrote the collection in London, idealizing the county from memory. The poems have a powerful sense of place — the hills, the orchards, the market towns — and reading a few before visiting sets the mood.