Planning a trip to Portugal this summer (2020). Besides downloading the current updates from Rick's website, is the 2017 edition still accurate, or should I purchase the 2019 edition? Thanks!
Like you, I have the 2017 edition. In looking at the RS shop, the description of the 2019 edition is 33 pages longer (493 vs 460) than the 2017. The "new one" isn't slated for release until March 2021.
The answer may depend on what you plan to do in Portugal. A lot of the information in your current book is probably still relevant, but it would be a good idea to check the guidebook section of the website for updates.
Of course, you'll get the best answer to this question by contacting the RS office.
I don't know what else was added, but I do know that Tomar is in the 2019 book but not in 2017. Tomar is worth visiting. I found the RS guide extremely helpful for the Knights Templar convento, which is a main attraction in Tomar. As I remember, the audio guides were not available or maybe not in English when I was there and the printed information was somewhat limited. The RS book pointed out what to look for in different rooms. I would have missed a lot without it. Lonely Planet, by contrast, had more on other things in Tomar but lacked the detail on the Convento.
The online Guidebook Updates primarily address info from older editions that no longer work (e.g. closed sites and changes to contact info). But they don't mention what's newly added (Tomar and a few other features), re-written, new maps, things deleted for reasons other than having closed, or the typical small details that get revised as you go. Rick was one of the researchers on the 2019 edition and he exercises more license to add and rewrite.