Libraries care about library catalogues, or, more broadly, lists of books, compiled for all sorts of reasons.  On the back flyleaf of a manuscript of a well-known grammatical text is a list of medical books:  it's in the same hand as that of the grammar, and it ends with a curious statement:
The sum of Bertykin, containing the summary of all that which is in the library of his own house, which was acquired in Nuremberg for six florins.

There are uncertainties in this reading:  is this indeed the sum of little Berty's books?  Or is "summa Berthikini" itself the title of another text?  If so, I certainly haven't found it and I certainly have looked.   And why is "comperatur" in the singular?  Are all the listed texts in fact bound as one volume?  Not impossible, since none of them is very long; they add up to some 120 leaves in early printed copies of the works.

Speaking of print, were Bertykin's texts in manuscript or in print?  The fourth and fifth item occur in this order and, more compellingly, with the same unusual title for the fifth text in an incunable printed in Rome by Bartholomaeus Guldinbeck ca. 1475-76 (?;Goff A-1070).  Even the main text, the grammar, Lilium grammaticae, could easily have been copied from a printed text; there were nineteen editions of it between ca. 1490 and 1501,  and the edition printed ca. 1495 in Cologne (Goff V-264) provides a good match with the present manuscript.  Or perhaps Bertykin owned a mixture of manuscript and printed texts:  the 6th item wasn't printed until 1919-1920, as far as we know.

What do we know?  That the watermark of the paper suggests a date of the main manuscript at the end of the 1400s (but the final leaf, the one with the book list, is on a different paper stock).  That the same person copied the grammar and wrote the list of books, and that person was German (paleography, quotes in the grammatical text, reference in the list to Nuremberg all agree).

Plimpton MS 137, f. 17v:   Bertykin's Books:

• Liber arnoldi prepositi S. iacobi de regimine sanitatis ad agustinum  episcopum sagabriensem [The Book of Arnold, provost of St. James, On the regime of Health, dedicated to Augustine, bishop of Zagreb]
• Liber qui dicitur Thesarus  pauperum in medicinis et incipit Practica dividitur [The Book which is called the Treasure of the Poor in Medicine, and it opens with the words, "Practica dividitur"]
• Tractatus de venenis Petri de Albano [The Treatise on Poisons by Peter of Abano]
• Tractatus Magistri arnoldi de villa nova de arte cognoscendi venena [The Treatise of master Arnold of Villanova On the Art of Knowledge of Poisons]
• Tractatus de Epidimia et peste domini Valasci de tharenta regis francie prothomedici excellentissimi <a price?> [The Treatise on Epidemics and on the Plague by Valascus de Taranta, the most excellent main doctor of the king of France]
• Et Flores dietarum Magistri iohannis de sancto paulo [The Flowers of Diet by master Johannes de S. Paulo]
• Summa Berthikini continens summatim omnia que habentur in libraria domui proprie comperatur Nuremburge pro 6 florenis.  [The sum of Bertykin, containing the summary of all that which is in the library of his own house, which was acquired in Nuremberg for six florins]

For more images and for a brief description of this manuscript, please see the Digital Scriptorium searching on the call number: Plimpton MS 137.