I should have quoted Read's book in the first place
It's the book I always suggest when it comes to notational issues. I didn't remember that it mentioned the 15ma bassa signs. Maybe I should re-read it quickly...
In fact, I find Read's book to be very very helpful - it says a few essential things about the development of notation, and then it lists the notations in quite a user-friendly way. However, I've found Kurt Stone's book on 20th century notation to be more up-to-date, and contain a bit more information on the details, like secondary beams on triplets and stuff like that. I think he actually mentions that Stockhausen has miscalculated the triplet-quintuplet-triplet's in one of his Klavierstucke, and they don't match up to the time signature of the bar... That says something about how much in detail the Stone book goes.
I haven't read all of it, though, just bits that the Read book wasn't clear enough. But still, I'd recommend the Read book over the Stone.
Weird surnames, all of them.
Now, enough of my little monologue, Qcc has spoken. And yes, cautionary/auxilary/courtesy accidentals are always more than welcome by the performers - we had a workshop the other day and the amount of time we spent discussing whether or not a note was flat or sharp because there wasn't an accidental on the score after a bar on a repeated note (or similar cases) is unbelievable..