Well, there's simply no alternative to Simon Moore's 2.9 book. And after seeing in another thread that Alleggerita is rocketting towards $1000 price ranges, albeit I rate Alleggerita an excellent book, I would be prone to conclude that the Immortal 2.9 is a bargain at the same price!
Indeed, Simon is currently working on two projects. One is the third part of his trilogy, now covering the GP cars of the thirties, and the second is the update of the 2.9 book. Indeed, much information surfaced until 1986, and a second edition would be really useful. Not sure which of the two projects will be completed first. Now think that back in 1986 he announced the 2.3 book within a couple of years...