The March painting on my Women Reading calendar (see also 1st of each month since) is Carl Vilhelm Holsoe's Girl Reading At a Sunlit Window. Holsoe was Danish, and lived between 1863-1935, although I cannot find a date for this particular painting.
What is she reading? A novel, I think, not a letter or a book of devotion; I think she is a modern young woman, serious and reflective, reading the best books of her generation, not frivolous or idling - there's no sexuality or mischief here. So, from my extensive knowledge of Danish literature, I offer the following suggestion - Niels Lyhne, by the novelist and poet J.P. Jacobsen (1847-1885),which was published in 1880, and which apparently inspired (among others) Thomas Mann, Rainer Maria Rilke, and the composers Delius and Schoenberg.
According to one (anonymous) review I found on the web, "Niels Lyhne recounts the life of a poet, emphasising the influence of experience on psychological development and examining philosophical issues: the nature of reality, atheism, creativity and love. It is a dense narrative, striking at times in its richness of physical detail, although the prose, which occasionally sparkles, is baroque and at times rococo. The book's appeal to early modernists and champions of realistic fiction is understandable, but this modern reader found that the characters never developed into living creatures and with few exceptions were too obviously created and manipulated to address the author's aesthetic and philosophical concerns."
It's available in Penguin classics - maybe we ought to read it? (Do let me know if you have, of course!)