Having bought Dead Letters (pop-rock) you may be considering The Rasmus' back-catalogue, and if you're feeling a bit adventurous I would recommend Playboys next (the safe option being Into). Don't bother too much with Hell of a Collection, because it doesn't show off their best work, and Playboys sums up their first three albums in a much better way. If you didn't like Dead Letters then I would definitely recommend Playboys, because this album will change any ideas you, or your friends, have about their style of music or their capabilities.
This album is their most relaxed (as proven by the introduction - listen to it in order when it is first played) and experimental, so is my favourite. It's fun (but not quite as fun as Peep) and contains some of their best songs. The huge variety of styles makes it so enjoyable to listen to: ska-punk, skater-rap, drum and bass, (one pop-ish song), reggae and rock throughout.
One of their most different songs ever is Wicked Moments, and they have never done anything else like it; it's a fantastic song. Ice is another particularly good track, from the different structure, to the melody and lyrics.
Carousel and Sophia are guaranteed to make you smile, although the latter takes some coming around to. Raggatip is the slowest, chilled song. I love all the other tracks in such different ways. The only slightly annoying song is Playboys, because it is more pop-y than the others on the album, and has a rather repetitive melody if you aren't in the mood.
You won't find another album like this, particularly as it is by just one artist. People who have overheard me listening to it always comment on how good it is (especially about Ice, Sophia and Wicked Moments), so are *extremely* surprised when I tell them who it is! I am totally in love with it, and I would like more people to discover it too!