WriteString bug???

It's simple enough:

Function RottbottReadString$(File)
  LenLen = ReadByte(File)
  For i = 1 To LenLen : SLen$ = SLen$ + Chr$(ReadByte(File)) : Next
  For i = 0 To LenLen - 1
    Len = Len Or (Asc(Mid$(SLen$, i + 1, 1)) Shl i * 8)
  Next
  For i = 1 To Len
    In$ = In$ + Chr$(ReadByte(File))
  Next
End Function

You could use a bank with ReadBytes() to make it quite a lot more efficient. The equivalent WriteString would be even simpler.