Schirmer/Mosel has recently released Volume 3 of the catalogue raisonné devoted to Cy Twombly's drawings, and Volume 4 is due to be released this fall:

CY TWOMBLY: DRAWINGS -- CAT. RAIS. VOL. 3, 1961-1963.
Ed. by Nicola Del Roscio.
Gagosian Gallery, New York, 2013. Published in association with Schirmer/Mosel, Munich. Distributed by Prestel-Verlag Munich.
216 pp. with 297 col. ills.
35 x 26 cm.
ISBN 9783829604871
Hardcover
In English
$195.00x
Worldwide 78977
CY TWOMBLY: DRAWINGS -- CAT. RAIS. VOL. 4, 1964-1969.
Ed. by Nicola Del Roscio.
Gagosian Gallery, New York, 2014. Published in association with Schirmer/Mosel, Munich. Distributed by Prestel-Verlag, Munich.
208 pp. with 286 col. ills.
35 x 25 cm.
ISBN 9783829604888
Hardcover
In English
$195.00x
Worldwide 79895
Details on the fifth and final volume of the drawings have not yet been announced. However, Worldwide welcomes series standing orders, and we can schedule to supply the final volume to interested libraries once the title becomes available.
Schirmer/Mosel has also issued catalogues raisonnés on other aspects of Twombly's oeuvre, with titles devoted to his paintings and photographs.
The sixth and final volume of the catalogue raisonné on Twombly's paintings is due to be released in October:

CY TWOMBLY: CATALOGUE RAISONNE OF THE PAINTINGS -- VOL. VI, 2008-2011.
Ed. by Heiner Bastian.
Schirmer/Mosel, Munich, 2014. Distributed in the U.S.A. by Prestel Publishing, New York.
192 pp. with 50 col. ills.
30 x 27 cm.
ISBN 9783829606882
Hardcover
Bilingual in English and German.
$298.00x
Worldwide 162182
Note: Volumes 1, 2 and 4 of the painting were released in the late 1990s but remain in print and can still be supplied by Worldwide as special-order items; Volume 3 appears to be out of print; Volume 5 was released in 2009 (see Worldwide 132504).
Volumes documenting Twombly's photographic oeuvre include:
Of related interest is the forthcoming title The Essential Cy Twombly (Worldwide 812010) as well as Cy Twombly Gallery: The Menil Collection Houston (Worldwide 77835).