The letter & Bank statement must not be over (7) days. Checking from the day you received it, and including the you go to the immigration office.
Your bank book as stated in one of the above post, must be updated on the day you go there. And copied.
Do not forget the house location map / couple of passport photos / Marriage certificate / wife change of name certificate, (if she changed it) / wife ID card / house book / your yellow house book if you got one / Child; Children Birth paper / Pictures of you wife & child / children in the ( Living Rm / Dinning Rm / Kitchen / One showing house number with you all / Bedroom ( note, all photos (2) per 8x4 page) / TM30 and/or last 90 days reporting slip / your passport detail pages copy / plus all pages in your passport with any Thai immigration stamp in it / Application form / Over stay : obligation form/ all copied checked signed / most importantly your application money / if this is your first extension, they would come knocking on your door, and want to meet your neighbors. Make sure you offer them a cold beverage.
Nonthaburi can be a rouge IMO.