I don't know about you but I divide books into two categories: there's books that exercise the mind and require an effort to understand, and then there's books that require no such effort. All the Richard Sharpe-novels for instance fall into the latter category, the books Patricia Finney writes into the former.
That is not to say the former category is any less good, on the contrary. With me it just depends on the mood I'm in: sometimes I'll just want to 'sit back, relax and enjoy the ride', and at other times the need to take notes, re-read passages to make sure I've understood them fully, look up references and generally sort of struggle with a book to have it release its full meaning adds to the pleasure.
But to return to "Firedrake's eye": if you make the effort this is a cracking good read. The plot is complicated to say the least (I'm sure surviving in a court as riddled with intrigue as Elizabeth's was too), the language dense and compact but by the same token poetical and highly charged with meaning, and Finney brings to life Elizabethan London in a very colourful way. If you liked "Firedrake's eye" by all means read "Unicorn's blood" and "Gloriana's torch" as well, they both are as good.