| I play in a band that plays both sambas and bossas - we have a Brazilian singer. But I think I tend to agree more with Ed, that Bossas can be a lot more fluid and follow the song/singer, whereas Sambas are more based around the Surdo rhythm and are therefore in stricter time.
Typically Bossas have more chord changes and are much lighter in feel than sambas. Mostly they are tunes written for just guitar and voice, which is why you won't find much about how to play bass "authentically" in this style. They were famously taken up by US Jazz musicians from Stan Getz onwards and have been developed into a variant of Jazz ballad style that is more rhythmic, but is now a sort of Jazz "fusion". So you can find them mentioned in Jazz tutor books and if you get books on Jazz bass, you will often have sections on "Latin" which includes bossas.
I often find that it's better to follow the singer rather than anything else on bossas and tailor the bassline to the way they sing - I think they are songs that are easy to play from a bass point of view, but not easy to play well.
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“Making the simple complicated is commonplace; making the complicated simple, awesomely simple, that's creativity.”
Charles Mingus | 
