I think you could write a decent book on Scala without knowing the definition of a Monad (indeed I think Martin Odersky's book [ which I really like ] only mentions monads once in passing when describing the most general form of the for comprehensions).
On the other hand I find it hard to imagine how you could have written a lot of Scala and not have realized how useful the flatMap function is (even I find that I use it frequently in practice, and I am a recalcitrant user of null pointers and at least 1% vars). While I am tempted to say that any book written by someone without a lot of practical experience is not worth reading, there is a real chicken and egg problem with relatively obscure languages in that there are not enough people who have used the language in their day job for years, and who are able and interested in writing a good book on it. So I think the answer to your question is YES, we should be getting in on this book writing game. Someone has to. Note that I am not volunteering... by "we" I mean "you". :-) Writing a book is hard.
> someone suggested that it was dishonest to even prepare for a
talk/presentation, because if you don't already know it off the top of
your head, then you don't know it well enough to present.
The talks I have gotten the most out of have been rambling "poorly" prepared talks by an expert on the field who lets me see how an expert in the field thinks. If I want a well prepared talk, I will read a (well prepared) textbook - I can read faster than someone can speak. But most students disagree with me. (I remember one class I had where the feedback from the students was "one person really liked it, but everyone else wished it was better prepared").
I will put forward another example - the Feynmann lectures in physics. They were well prepared, by one of the best in the field, and were generally considered as excellent and fascinating by other experts in the field - but the students doing the actual course did poorly. So maybe we should just be happy that there is another Scala book out there, and who knows, it may turn out to be good for some people. If not, then it will probably fade into obscurity and better books will be written and little harm done.