I have got a copy of Grow Your own Veg.
I find it very useful to dip in and out of and it has sensible advice.
I like a reference book to be either subject specific or time specific. GYOV is of the former. You can check out any particular veg.
I also have The Allotment handbook by Caroline Foley which is useful reference. It has a directory of vegetables but also goes through the calendar year which I find very helpful.
