For "The Curse of the Voynich", I forensically examined the manuscript itself, travelled to all the places, critically read all the secondary sources, and from all that reconstructed the story as best I could. In short, I'd done an OK job: but though readers told me they liked it, it hadn't set the world on fire. Though it ticked all the right boxes, it was obvious I had to go away and try harder. But what could I do better?
At first, I bought a pile of books on the history of cryptography, such as David Kahn's "The Codebreakers": all fascinating, but the question I'm trying to answer - "at the Quattrocento birth of mathematical cryptography, what kind of cryptography died?" - only features marginally (if at all) in the generally rather positivistic accounts presented there.
And then I realised what I had been missing. Sure, I had read plenty on Quattrocento individuals such as Filarete, Alberti, Brunelleschi, Taccola: but there was one gigantic motherlode of information which most historians seem to pay lip service to (rather than have to set aside several months to read): Lynn Thorndike's epic multi-volume "History of Magic and Experimental Science".
I therefore bought volumes III and IV (for the 14th and 15th centuries) and have now reached halfway through the latter. What continually amazes me is the amount of ground Thorndike covered that has apparently not been touched by anyone since: though there is a large literature tree cascading off it, it is very deep in places and non-existent in others.
From what I have read, I am now quite sure that virtually all of the Voynich Manuscript's roots will turn out to be directly traceable from the late 14th and early 15th century: which means that we might in time be able to reconstruct or predict plaintexts for some sections. But these are still very early days in this ultra-long-term research programme. *sigh*
However, the good news is that I also bought a copy of "Science and Thought in the Fifteenth Century" by Lynn Thorndike: and one obscure page from that gave me precisely the clenching-both-fists-in-the-air-YESSSSS-moment I mentioned at the start. The details are too convoluted to go through here, but trust me, it's a peach.