Hi,
All depends on who you talk with. I've heard one or two neg comments about Darryl and his company, but wrote those off to jealousy or just plain old bile.
Overall, a few parts I've bought either directly or indirectly from Racetorations have been excellent, but that hasn't yet included anything so large (and expensive) as the billet crankshaft. Racetorations does provide a number of things that are innovative and vintage road race/sprint oriented. (Revington and Cambridge tend to be more rallye oriented.)
I believe DKU offers both: a billet crank that is a close copy of the original and another that "straightens" out the design using several dummy mains. The "straightened" crank shoudl spin up faster, due to lower interia. It makes sense to me and seems a clever idea, but unfortunately I don't know of anyone using it. As soon as I win the lottery, I do plan to try one in my TR4!
I think Darryl's crankshafts use standard flywheel fitment. I am pretty certain some others, perhaps from Cambridge Motorosport and Revington TR, require a special flywheel. Best to ask about this from any of the vendors, before commiting to a crankshaft (unless you are also planning a flywheel swap).
Roger Williams' "How to Improve" books are very good overviews (complementing both his own"How to Restore" books and Bill Piggott's "Original" series). They are relatively up to date with current supplier info and a lot of neat ideas. Don't look for deep detail and lot's of engineering. The books are mostly of an overview and compilation of ideas from mostly British sources (Williams really needs to interview folks in the U.S. for his Second Edition revisions!), and are part of Veloce Publishing "Speed Pro" series. Amazon always has them, and they often appear on eBay too.
There is no book (yet) for Spitfire & GT6, but the TR5/250/6 edition will talk about the engine you are using.