If you want a record of messages sent and received, set
jabber-history-enabled to t. This will save all messages to
the file specified by jabber-history-file, by default
~/.jabber_global_message_log. There is no facility for
reading old messages yet, but just reading the file as text should be
enough for many purposes.
When you open a new chat buffer and have entries in your history file,
the last few messages you recently exchanged with the contact in
question will be inserted. You can control how many messages with
jabber-backlog-number (by default 10), and how old messages
with jabber-backlog-days (by default 3 days).