I am sorry to say I know very little of Winter War sources in English, there's a wealth of them in Finnish of course...
One book is The War of the White Death by Bair Irincheev, published by Pen & Sword in the UK. I guess you could say it's more of a general history on Winter War, depicting the events more from strategic level, but there's mixed in some stuff from soldier perspective, and interesting photos and maps. It's fairly balanced view I think, based on both new and old sources. I dont' know the exact background of the author, but I know he's into organising guided tours to the Mannerheim line etc. so he's at least keen enthusiast, and should know his stuff. It doesn't hurt that he writes also from Russian perspective, he tends to give a little different emphasis to some events and is critical of both Finnish and Russian sources. Certainly a good read in addition to whatever sources you might be able to get.
Another source that concentrates on Finnish uniforms is Asepuku m36 by Petteri Leino. This may be more difficult to get, and it's mainly in Finnish, but contains summaries of the texts and captions in English. The main value is color photos of uniforms though, good painting resource. Some interesting b/w photos as well.