The publication by Hipólito Sancho de Sopranis titled "//La colonia portuguesa del Puerto de Santa María. Siglo XVI. Notas y documentos inéditos"// can be described as a brief treatise.. it provides specialized historical analysis and previously unpublished documents related to the Portuguese community in Puerto de Santa María during the 16th century. The concise nature of this work makes it particularly suitable for scholars and readers interested in targeted historical studies without the extensive length of larger volumes.

This is what the dictionary says-
https://www.vocabulary.com/dictionary/treatise
A treatise is a formal written paper about a specific subject. It’s like an essay but longer. A treatise is usually about a serious subject, so you might read a treatise on democracy, but you probably won't read a treatise about chewing gum.

https://www.merriam-webster.com/dictionary/treatise
1 : a systematic exposition or argument in writing including a methodical discussion of the facts and principles involved and conclusions reached

The Latin term seems to be common in English. Its plural is "opuscula".