Elena Caravela is my most favourite contemporary portrait artist. I greatly admire her ability of seeing colours in hairs and eyes. She can capture the spirit of her subjects and fleeting moment of expressions like magic. I love her children book illustrations too.
Finnish National Gallery collection includes hundreds of sketchbooks different artists. They are grouped by artists on the web page by the link above. It is a great privilege of seeing individual artists’ artistic development through their sketchbook.
The National Portrait Gallery of the UK collection consists of over 200,000 items with a wide range of media and styles from the 16th Century to the present day is also a great source of reference materials.