I highly recommend the Hal Leonard "Brazilian Piano: Choro, Samba and Bossa Nova" tutor book.
Although it is written for piano, often the bass is simply single notes, as it follows the Brazilian bass drum ("surdo") and so, is appropriate for accordion.
The styles covered are Choro (the precursor of samba), Samba, and Bossa Nova (a Brazilian mix of samba and jazz). The roots of samba and choro began in the north of Brazil with the rhythms of African slaves. Then in Rio de Janeiro the mix of African rhythms and Portuguese melodies deepened in the early 1900s.
In the tutor book, within each style, especially samba, there are a myriad of (syncopated) rhythms that follow the various beats of the cowbell, tambourine, and other percussion instruments.
All this you can learn to play on accordion! In Brazil, Choro and Bossa Nova music is sometimes played on accordion, but Samba almost never.
Now, I'm just waiting to find a tutor book for the fantastic, North-East Brazil accordion-centred styles of Forro, Baiao and Xote... I've never seen that before though, as it was and remains a very poor, un-industrialised region of the country, I'm not sure if there have been any published. It seems that the North Eastern styles really are aurally handed down.