If you've read those books and like them, the logical thing to do now is to rewind back to Book 1 - THE COLOUR OF MAGIC and proceed in publication order. This is normally a tricky rec as the earlier books are not as strong as the later, but if you've already dipped your toe into the later books and know they get better, that shouldn't be a problem. Reading in order is also helpful because whilst the books are divided into sub-series and stand-alones, the links between them become quite complex later on (for example, the guards showing up as supporting characters in some of the stand-alones and wizard books).

If that doesn't appeal, the logical next step would be MEN AT ARMS, the sequel/follow-up to GUARDS! GUARDS! in the 'Guards' sub-series, or REAPER MAN, the follow-up to MORT in the 'Death' sub-series.