hope to get an explanation of what happens to a text message when I
send it but my friend doesn't receive it or receives it a day or two
later. Is someone else able to read these lost messages?

according to the info that i have, no other user can read the messages
because they cannot have an access to the same. but the messages
remain in the servers of the mobile service providers and can be
retrieved and read if so desired.

so the answer is part 'yes' and part 'no'

When a message is sent from one phone to another it is sent to the
SMSC (short message service center) in the mobile network providers
network.  The SMSC attempts to deliver the message to the recipient,
if this fails because the phone is off or out of coverage the SMSC
will retry the message later.  During this time the message is stored
in the SMSC.

Theoretically the message can be read by someone with access to the
SMSC but with thousands of messages in this retry status they would
have to specifically search for your message

for example:  If a friend and I were in the same building, no farther
than 30 ft apart (both in the cingular service area w/good reception)
and he and I were texting each other. Two out of nearly twenty of the
text messages that I sent to him, he did not receive at that time. A
day or two later, he recieved several messages at once from my number.
The messages he recieved were messages that I had sent but not all of
them were messages I had sent to his number, some were messages sent
over a month earlier to another persons cell phone. I am confused
about this.