For comprehensive Istriot grammar resources, focus on academic works from the late 19th and 20th centuries, particularly those by linguists like Antonio Ive and recent scholars. The most thorough grammar is likely "Il dialetto istrioto" by Antonio Ive (1900), which covers phonology, morphology, and syntax, though it is in Italian and may be hard to find in print. For modern research, search for doctoral dissertations and journal articles in linguistics databases, as they often include detailed grammatical descriptions.
To start, check the online catalogues of university libraries in Italy (e.g., University of Trieste, University of Udine) and Croatia (e.g., University of Rijeka), which may hold digitized versions of rare works. Google Scholar and the open-access repository Academia.edu can yield downloadable PDFs of recent papers. The Endangered Languages Archive (ELAR) at SOAS University of London might have field recordings and grammatical sketches. Be aware that resources are mostly in Italian or Croatian, and many are out of print or behind paywalls; you may need to request interlibrary loans or contact authors directly.
Tradeoffs: Older grammars are authoritative but may not reflect modern linguistic analysis or account for language change. Recent dissertations are more current but can be narrow in scope. No single resource covers everything; you will likely need to compile information from multiple sources. A practical next step is to join the Istriot language community on platforms like Facebook or Reddit, where speakers and researchers share leads. Also, consider reaching out to the Croatian Academy of Sciences and Arts or the Institute for the Croatian Language and Linguistics for guidance. For a broader Romance linguistics perspective, resources on Venetian or Friulian can provide comparative context, as Istriot shares features with these languages. If you are also learning Dutch, our guide Dutch Fluency may help with other language projects, but for Istriot, persistence and academic networking are key.