jts wrote:2. Was your estimate that a book is (++) premised on the reader giving the book one thorough reading, or on fully understanding the material in it?
3. I'm assuming that most of the (++) are only valid if it's your first time... clearly there are diminishing returns to, say, knowing more tesuji. If you think the book you're recommending is (++) even if you've already studied the topic a little, I'd be curious to know that.
My previous comment is related to these questions 2) and 3). First of all, IMHO, reading a book may do nothing at all for your playing strength. In fact, initially it may make it worse as you try new things you saw in the book but don't yet understand. By the time you understand, it may be hard to trace back the improvement to any given book, especially if you are doing other things like playing or getting advice from stronger players. I never read a book and then put it down and said: "aha, now I see the light" and immediately started playing better. I have read all of the books on your appended list except for Robert's and Sakata's, plus many more that aren't listed. That being said, like a trumpet player always looking for the perfect mouthpiece, I still dream of seeing such a book someday and so I probably will check out Robert's and Sakata's.
My playing strength has always been highly correlated with my reading ability at the time (as measured by the difficulty of tsumego that I can solve). It's painfully, unforgivingly correlated and often it seems there is not a damn concept or strategy or philosophy in the world that has ever seemed to break that. For me, only improvements in reading improve rank.
But I may be a really bad example because I do study. People who never study at all but just play may be able to benefit from an occasional book. I'm not rejecting the idea that ++ books exist for some people. I just haven't had that experience. I hope I'm not alone.