Her Wikipedia entry is here and a blog post containing biographical information and plenty of images is here.
The best place to view Schjerfbeck's paintings is the Ateneum in Helsinki where, if memory serves, a room is devoted to her works.
I have trouble evaluating modernist painting because I care for little of it. I'll simply mention that I think her best modernist paintings are those that don't stray far from realism. Here are a few to provide a taste of where her style evolved.
She painted some landscapes.
Just enough modernist traits to make this an interesting mostly-representational piece.
One of her last self-portraits.
Below are examples of her early paintings, most or all of which were made during the 1880s when she was in her twenties.