Second, third and fourth book of the Courtier, by Baldassare Castiglione in Latin...

This manuscript copy is a continuation of The Book of the Courtier by Baldassare. The text is a Latin translation of the second, third and fourth books. The second book is written in the same very neat hand as the first book in HMS/4/15 and the second book in HMS/4/16. The third and fourth books are in several different hands, some of which match the hands in the English version of this manuscript (HMS/4/16). There has been some suggestion it may have been William Cavendish who wrote this manuscript (later the 2nd Earl of Devonshire), but there is no evidence of this from the manuscript itself apart from corrections to the text throughout and uneven line-spacing which might not occur in a scribal copy. The fact that the hands match those writing the Latin manuscript (HMS/4/17) and that the end date of both manuscripts is a day apart suggest the manuscripts were probably produced by the same person. There is also a reference in HMS/1/15 to Mr William Cavendish being paid £5 by his father for finishing his Latin and English translations of the Courtier, as pointed out by Timothy Raylor.
At the end of the manuscript is written: "Finis quator librorum Aulici being the 4th day of October in the third year of the reign of our Sovreign Lord King James 1605 - Clauditie iam rivce pueri sat prata biberunt."

HMS/4/15, HMS/4/16 and this item were clearly a set.

The Italian author, courtier, and diplomat Baldassare Castiglione (1478-1529) is known primarily for his "Book of the Courtier." This work, which portrays the ideal courtier, was a chief vehicle in spreading Italian humanism into England and France.

Source: 'Baldassare Castiglione', Your Dictionary.com, https://biography.yourdictionary.com/baldassare-castiglione [accessed 8 Jun 2021].