This is a very old thread that Delimbeau has re-opened and I think that the answer is extremely subjective. It depends how you define "value"; whether it be in respect of content or monetary worth. If the former, there are probably as many answers as there are books but if - as I think was the original ask - it is purely monetary there are only a few candidates, most of which have already been mentioned.
First let's ditch Scot's "Discoverie of Witchcraft". Whilst the second issue of the third edition is likely the scarcest, the first edition will almost always be more valuable. Aside the magic content, this is a very desirable book in many fields and therefore always commands high prices. It is not, however, rare. As an update on price, copies of the first edition in the last couple of years have reached up to c. $70,000. Compare this with the aforementioned third edition, a copy of which a couple of weeks ago sold for "only" $14,000 + commission (in itself a high price).
The Guyot, Dean and Pinetti mentioned in the thread we can also discount, as "comparatively" they are of little value (in any edition).
Of known books on magic, likely "The Art of Jugling" (1st ed. 1612, 2nd ed. 1614) or "Hocus Pocus Junior" (1st ed. 1634, 2nd. ed 1635) would be the most valuable. "The Art of Jugling" was the first book solely devoted to magic in the English language (albeit plagiarised from Scot) and only one copy has been offered for sale in the lifetime of most people who will read this. The six-figure dollar asking price would exceed any copies of Scot's work I know of. Of course, "asking" and "sale" price will not necessarily be the same.
Recent (i.e. last 10 years or so) copies of "Hocus Pocus Junior" have "only" reached the c. $36,000-$60,000 price range. These have been later editions. That said, a first edition certainly would achieve a price well above that, if one were to emerge - unlikely since none has been seen since the 1930s. In the event that it did, it may well exceed those prices recently realised by Scot first editions.
Probably the most valuable book on magic is one that we do not yet know exists, or suspect that it does but a copy has never been seen. For example, some may be aware of Prevost's "La Premiere partie des subtiles et plaisantes inventions". It is a French illustrated book devoted solely to magic that predates Scot (by only a few months). As it is not in English it doesn't get as much attention but is arguably much more exciting from a purely magical context. For those French readers, you will note that Prevost's book was the "first part" (i.e. premiere partie). If a second part were to emerge, I would warrant it very valuable indeed.
The potential "second part" of Prevost is but one example. Lost manuscripts or other early works (most likely in Italian) may also compete. The fact is, though, that regardless of content the majority of the market is English speaking, so early English works are likely to continue to command the highest prices, regardless of their rarity or importance. We must also remember that our field of interest is very niche. Another book with magic in it but with much wider appeal would attract more attention and - potentially - command a higher price; i.e. the high prices seen recently for first edition Scot's are predominantly NOT due to magic collectors but collectors with other fields of interest.
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