Would like to learn Twi

Lucky you ! :-) I have been in Ghana and it's a marvelous country, you'll never want to come back !!

For your question  a great book I came across is "A COMPREHENSIVE COURSE IN TWI ASANTE" by FLORENCE ABENA DOLPHYNE (Ghana University press, ISBN 9789964302450).Although it's not sold with a tape or CD  as it should (to help dealing with pronunciation) it's still a great way to learn, especially  the grammar section. Very helpful, clear, packed with examples taken from current everyday usage etc... Don't  miss it !

Another  useful tool is the "TWI-ENGLISH / ENGLISH-TWI DICTIONARY" (Hippocrene Books, ISBN 9780781802642). Pocket-sized, very practical.

BE CAREFULL !! Twi is a dialect among others.  You therefore have different kind of Twi (Asante, Akuapem and Fante), each with their differences in vocabulary and syntax. These differences are slight, but if you want to  learn it try to stick to one variety (Asante is the the most widely spoken).  If you have Ghanaian friends to help you out (the pronunciation is  tricky at times!) it would be even easier :-)

Check on Africabookcentre.com. Dedicated to Africa they have  good references  for books  about Ghana but also about african languages (including Twi). Some good ideas to pick up!