At last night’s joint user group meeting for the Windows Server UK User Group and the Active Directory UK User Group, James O’Neill mentioned that the book he has co-authored (Microsoft PowerShell, VBScript and JScript Bible, published by John Wiley and sons) goes on sale today.
I haven’t had the chance to review it yet but knowing how immersed James is in PowerShell (and that he wrote the PowerShell sections), I would suggest this might be worth considering if you are looking for a good reference book.