The Wellcome Collection in London is a valuable resource for any researcher in the history of medicine or science. However, recently I was directed by a member of staff to take a look at http://images.wellcome.ac.uk/. This is a truly superb search database that brings up digitised images in the Wellcome Collection.
In my case, searching Alfred Russel Wallace brought up a series of digitised letters to and from Wallace. Two have so far caught my eye. One in which Wallace directs one correspondent to good texts (including his own) on the subject of spiritualism and another in which he gives advice about health to a mother whose son is about to head to the tropics.
There really is so much here to work on. Do comment on anything else you come across.