The Line
The railway itself still exists in preservation. Their website contains plenty of information for the current era. They have a quarterly magazine and the back issues are a great resource of historical information.
Books
The Welshpool and Llanfair Light Railway by Glyn Williams is probably the definitive work on the railway prior to preservation. It has countless pictures and a significant amount of text covering the decision to build the line, the various routes proposed and the eventual building, running and decline of the railway.
For the modeller there are numerous track plans along with some relatively detailed drawings of the various rolling stock used on the line
The Welshpool and Llanfair by Ralph I. Cartwright is a good read covering the entire history of the line from 1903 to 2003
There was a previous work by Ralph Cartwright and R. T. Russell called "The Welshpool & Llanfair Light Railway".
Narrow Gauge Branch Lines brought out "Welshpool to Llanfair, Featuring the lost connection"
Hugh Ballantyne wrote "The Welshpool & Llanfair Light Railway"
There is a relatively recent book of high quality photos of the line but I've mislaid it and can't remember the title.
Blogs
There is an excellent blog called Fifteen FlatOut which covers life on the preserved railway.
I'll add my own blog in here simply as another way to get to it.
Photo Sites
There is a superb set of pictures on Flickr by the same author of Fifteen Flatout. In Sep 2017 he put up a couple of pictures looking towards the mill from the opposite bank, a vanatage point not used before, at least not published online.
CoastRadar - has several pictures of the area around the Mill.
Your local web - again several pictures of the surrounding location.