track numbering in itunes, won't respect track numbering order

I have tried to import audiobook tracks created by ripping from my CDs. sometimes ITunes does not respect the filename or numbering and as a result the tracks are all out of order in the library or on the IPOD.they won't play in proper sequence. I've tried renumbering, retitling, and reimporting but Itunes sometimes will not respect that. how can I get ITunes to respect (and keep) the filename and numbering when importing so that the tracks are in proper order? an example are tracks originally numbered 1a thru 1x, 2a thru 2x, etc (corresponding to disc number) and they end up as in the order 1a, 2a, 3a, etc. can I renumber them in itunes itself, is there some kind of numbering convention?