The book that is constantly by my side at the moment is Alison Plowden's "Elizabeth I" which contains 4 parts that were orginally published as 4 separate books - "The Young Elizabeth", "Danger to Elizabeth", "Marriage with My Kingdom" and "Elizabeth Regina". Other books I'd recommend are Alison Weir's "Elizabeth the Queen", David Starkey's "Elizabeth" and "Elizabeth I (Women in History)" by Anne Somerset.
There are lots of books on Elizabeth I so other people may have further recommendations.
