In slipcase.
v. 1. Essays and references
v. 2. Paintings on canvas and panel
v. 3. Collages and paintings on paper and paperboard.
OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Robert Motherwell, paintings and collages : a catalogue raisonné, 1941-1991 by Jack D. Flam. ISBN 9780300149159. Published by Yale University Press in 2012. Publication and catalogue information, links to buy online and reader comments.
