As Kurdish_Assass1n intimated, you have several options. Which one is most efficient depends largely on the number of nicks you are keeping track of.

If you have your heart set on using $read, go with the previous replies. I myself never use $read, for no particular reason, so no doubt this has influenced my suggestions. None of the following suggestions involve mIRC actually reading whtlist.txt.

I'm assuming here that you merely want to know if a person messaging you is using a nick that is in your whtlist.txt

If you white list is very short, and you expect it to remain so, you might simply put the nicks in a variable and use token commands. This approach can be very limiting, tho.

For a not-huge white list, you might consider just adding those nicks to the User file with a level higher than 1 [e.g. 5], and use:

Code:
on 5:NOTICE:*:?: { echo -a $nick is on list }

OR

Code:
on *:NOTICE:*:?: {
  if ($ulevel == 5) echo -a $nick is on list
  else echo -a $nick is NOT on list
}

Be aware that this will trigger every time a level 5 (white list) user says anything in a message window. If you only want this echo when a private conversation begins, use on *:OPEN:?:*: { etc }

If your white list is quite large, you should look into hash tables.