Isles of Scilly Pilot by Robin Brandon published by Imray Laurie Norie & Wilson is quite good, my copy is old (1993). I expect there is an update. There is a another sort of Pilot written by a local which some people like, I found it a bit irritating, its available in some West Country Chandlers.
Dont read the pilot books they will frighten you ! It's absolutely fine when you get there, lots of mooring buoys if you want and you can see where all the lumpy bits are during low water. But dont tell anyone cos they'll all want to go now.
Just sent the proof copy back to printers and if all goes well should be back from the publishers sometime in March.
Its called the sailors guide to the Isles of Scilly.
The RCC (Imray) one is currently being updated and is due out in May.
Norm's yachtsmans guide is no longer available and is also a bit out of date.
If you happen to be a first time visitor and conditions are a bit fresh on arrival I would like to think that even in these days of electronic navigation having a book showing good photos of the marks for each entrance would give any skipper a better feeling of confidence during his approach