I remember the shock, like a jolt of static electricity. One day, between taking my degree and beginning my first job, while looking through a 16th-century book about numerology that had once belonged to John Dee in the British Library, I came upon an annotation in his own neat italic hand casting up the numerical values of the letters of his name. The total he wrote down came to 666.

John Dee (1527–1609) was a magus, but we must not think that this made him a loony witch. An early Fellow of Trinity College, Cambridge, teaching Greek, he acquired a reputation for learning in mathematics, navigation and astronomy. But his long pursuit was of something he knew was dangerous and which I am not convinced he always thought licit: angelic conversations.

He explained his intentions in a little speech to the Holy Roman Emperor Rudolf II on a visit to Prague in 1584, saying that he had spent 40 years in study ‘to come by the best knowledge that man might attain unto in the world’, but had found that no book could teach him the truths he longed for, and so he had prayed to God, whose ‘holy Angels for these two years and a half, have used to inform me’.

It was a notion widely held by Neoplatonist intellectuals in Dee’s time that the running of the cosmos was in the power of beings with intellect and will but unencumbered by bodies: angels. The trouble was that angels did not simply appear and impart knowledge to Dee. He had to interrogate them by means of a skryer, someone who could see things in a polished obsidian mirror or ball of crystal. The best skryer was Edward Kelley, who dictated endlessly complicated instructions for constructing tables in the Enochian language (spoken before the fall of mankind).

These angelic conversations went on for more than two decades, at first when Dee was trying to interest the Emperor and the King of Poland in the transmutation of base metals into gold. It might be expected that Kelley was a con man taking advantage of Dee. But it was Kelly who was frightened to go on. The fear was that the messages came not from benign angels but from wicked spirits intent on the damnation of these arrogant magi. What really rattled Kelley was an instruction that he and Dee should have their wives in common. Dee accommodated even this, while expostulating indignantly in his diary when Kelley was refused absolution from a Catholic priest who insisted that the spirits were evil.

This strange world must be borne in mind when considering John Dee’s astonishing bibliomaniacal achievement. He had set off for Prague with a mere 800 books packed up in crates. At home, in five rooms of his old house in Mortlake, he had left the biggest library in England, far bigger than those of Oxford or Cambridge universities or any of their colleges. Before travelling, he catalogued 2,292 printed items and 199 manuscripts.

Most of these manuscripts and about a tenth of the printed books have since been traced. The single largest collection of printed books is owned by the Royal College of Physicians, now showing off the cream of 100 or so items. Supporting objects include a crystal ball and a disc of gold carefully inscribed for the conjuring of spirits.

The Royal College does not brag about how it came by the books. The fact is that while Dee was abroad they were stolen from his house by Nicholas Saunder, an apostate Catholic and Member of Parliament. In a raid on the house, much damage was done and hundreds of books taken. Quite likely, Dee’s reputation as a conjuror played a part in the vandalism, about which he complained in a letter to Queen Elizabeth. From a sea captain called John Davis, Dee repossessed scores of books. From Saunder he extracted not much more than his ‘great sea cumpas [sic]: but without a needle’ in 1592.

Saunder was clearly dishonest, for he falsified his ownership signature on many of the books. The RCP has a volume of Quintilian, with Dee’s signature on the title page ‘Johannes Deeus 1545’ overwritten ‘Nicholaus Saunder 9 Decemb 1582’ (a date before the raid), with Dee’s name incorporated into the ‘Decemb’. Some ownership signatures were simply bleached out.

The Royal College of Physicians was not a fence to Saunder’s stolen goods but received them in an honest legacy in 1680 from the 1st Marquess of Dorchester, an amateur natural philosopher and the first honorary fellow of the college. It has proved a reliable custodian of this learned flotsam. F.R. Johnson, an authority on Renaissance astronomy and a champion of Dee’s scientific standing when he was still widely regarded as a superstitious fool, wrote of his books in 1937: ‘This great library was always at the disposal of Dee’s fellow scientists among his friends and pupils. If one believes that the first essential and true centre of any university is its library, Dee’s circle might truly be termed the scientific university of England during the period from 1560 to 1583.’

On show are lovely books such as a Euclid with pop-up geometric solids or a folio of Ptolemy’s astrological work, the Quadripartitum, printed in Venice in 1519 and heavily annotated by Dee. All the time, though, we must be aware that this stupendous bibliophile hoped for a short cut to a unified key to the cosmos. This could come not just from angelic conversations, but also newly derived from a sort of virtual computer-generated knowledge machine developed by the 13th-century Franciscan Ramon Llull. Dee possessed dozens of manuscripts and printed volumes of Llull’s works. At their heart was a method of generating knowledge via countless combinations of fundamental properties of the cosmos according to a variety of relations, at nine levels of existence. The combinations could be made by means of concentric dials, sometimes represented by paper or vellum volvelles rotating round a pin — like a medieval version of the Bletchley bombe. It may sound mad, but Leibniz for one took Llull seriously.

The Royal College of Physicians has Dee’s copy of the De Auditu Kabbalistico, purporting to be Llull’s work on the mysteries of the cabbala. The book was in fact written 200 years after Llull’s death, but bears witness to continuing Renaissance fascination with his thought. If ever there was a Renaissance man, John Dee was it, and we must take the angels with the Almagest, and the cabbala with the Euclid if we’re to understand his world.