Hello Daniel , here are the answers to your questions ... Here is the list
of the valid  Harpa species : crenata , doris , amouretta , gracilis , major
, ventricosa , articularis  , davidis , costata , harpa ,  kajiyamai . The
austroharpa species are : punctata , exquisita , wilsoni , loisae , learorum
. Harpa fulvomichaelensis is nothing but one of th many forms of H. major
coming from the eastern Indian Ocean . H. goodwini actually  is a form of H.
kajiyamai . H. kawamurai is a synonym of  H. major . Good quality specimens
of  H. costata var. laetifica and H. gracilis are extremely difficult to
obtain so I wish you :" Good luck ! " . Beware of  extremely high prices
which are far beyond the actual rarity of these two species , however .  As
far as the Harpidae literature is concerned  the best book about Harps is
still - according to me - " The Family Harpidae of the World " ,
Indo-Pacific Mollusca , Vol. 3 , N° 16 by Harald A. Rehder  , published in
1973 . Otherwise there is a very good study of the genus Harpa by  the late
Jan Berkhout which was published in Vita Marina Volume 41 N° 4 in 1992 and
... I also committed [ ;-) ]  one in Xenophora  N° 50 ( April-May-June
1990 ) . Poppe' s book can be useful  provided that you know  that the
colors of the photos do not always match reality   and that there are errors
so you should be cautious ... I am personally sorry that you are not
interested in Morum which indeed are a fascinating group ... Franck Frydman