Yay for Death of Rats! If you liked the style of ‘Soul Music’ a good next move would probably be ‘Moving Pictures’, another stand-alone novel about the magic of movie-making which on the Discworld turns out to be pretty dangerous. Also starring CMOT in an important role ^^
‘Making Money’ is possibly the only novel I did not obtain yet, mainly because it got bad critics overall. There are only very few of these somewhat ‘uninspired’ episodes of Discworld. Personally I could add ‘Mascerade’ and ‘Mac Best’ here but I would have difficulties finding any other novels that do not match the very high standards you can expect of Pratchett.
If you think about reading on be advised that the ‘series’ are best read in chronological order, i. e. normally in the order they were released. It makes understanding easier and you will find many allusions to earlier events.
My favorite characters are the Librarian (who doesn’t like him? ^^), Rincewind, CMOT and Cohen the Barbarian.