It depends on what you are after. If you've not made any stairs and just need the basics, up to and including stuff like standard kite winders and basic laminated curved strings then the standard C&G texts (e.g. Peter Brett's "Carpentry & Joinery" series) are where you should be going. If you've got a few stairs under your belt then one reasonable text is "Modern Practical Stairbuilding and Handrailing" by George Ellis which is a reprint of of pre-WWII treatise on the subject, however whilst concise, it makes some assumption of prior knowledge on the part of the reader. Not surprising, really, as some of the stuff Ellis deals with are quite complex. For handrail building there is a useful section on forming curve on curve work in Eric Stephenson's "The Spindle Moulder Book" (again Stobart-Davies) and one of the few places I've ever seen the machining of 3D curved stair handrail elements explained. There's also a new book on the market (to me), called "Simplified Guide to Custom Stairbuilding and Tangent Handrailing" by George R diChristina (again by Stobart-Davis), but as I've yet to track down a copy I can't say from my own experience how good it is (although it has had positive reviews - and how much can you trust a reviewer on a subject as technically obscure and complex as this is potentially the issue. BTW I have the other books mentioned in this reply and I have built a few stairs, other than straights, in the past)