To find books on the woman you've been assigned, begin in the Advanced Search area of the Library's discovery system with a search in subject, as follows: in subject is (exact) Fell, Margaret
This last name, first name pattern should work fairly well in subject is (exact) for modern names. Earlier names, though, can be trickier. Take Gertrude of Helfta, for example. She (unlike Catherine of Siena, who appears in the discovery system's in subject field fairly straightforwardly as Catherine, of Siena, Saint, 1347-1380) should be entered as follows: in subject is (exact) Gertrude, the Great, Saint, 1256-1302, or, for starters, as (since the comma makes no difference to the search engine) Gertrude the Great:
Notice that in the record above the word Helfta does not appear, and that, for that reason, not even a Keyword search on Gertrude and Helfta alone would have returned it.
That said, simple Keyword searches (in this case an any Any contains search with NOT "the Great") do often return relevant material that would have been missed by a proper Person as Subject search, and should in any case be used to find out what the official Person as Subject (or Author) heading (just covered above) actually is:
