"An inquest held by the king's escheator or his deputy after
the death of a tenant-in-chief of the Crown to establish the
extent of the estate and to confirm the rightful heir. A jury of
12 local men of high repute gave information under oath. Records
from the 13th century onwards are kept at the Public Record
Office, under C 133-142 and E 149-150. Calendars and indexes are
available for many reigns, and some county record societies have
published detailed calendars, especially for the period
1270-1350. They are arranged to a fairly consistent format, and
as well as showing what estates are held in chief by an
individual, they can be used to illustrate broad contrasts
between places and regions."
Oxford Companion to Local and Family History. David Hey
Oxford University Press 1996
More information on pages 506-508 of Ancestral Trails by Mark D Herber published by Alan Sutton 1997. This points out that the County Palatinates (Lancaster, Chester Durham) are held elsewhere in the PRO. It may be noted that the IPMs for the Palatinate of Durham do not cover land outside Durham.
More information at http://www.medievalgenealogy.org.uk/guide/ipm.shtml .
Henry III (1216-1272)
Vol I
(20-56) Henry III
http://www.archive.org/download/cu31924011387796/cu31924011387796.pdf
Calendar of Inquisitions Post Mortem, Volume 1
(fully digitised from british-history.ac.uk)
Edward I (1272-1307)
Vol II Edward I
http://www.archive.org/download/cu31924011387804/cu31924011387804.pdf
Calendar of Inquisitions Post Mortem, Volume 2
(fully digitised from british-history.ac.uk)
Vol III Edward I
http://www.archive.org/download/cu31924011387812/cu31924011387812.pdf
Vol IV Edward I
http://www.archive.org/download/cu31924011387820/cu31924011387820.pdf
Edward II (1307-1327)
Vol V
(1-9) Edward II
http://www.archive.org/download/cu31924011387838/cu31924011387838.pdf
Vol VI Edward II
(Part only)
http://www.archive.org/download/calendarofinquis62grea/calendarofinquis62grea.pdf
http://www.archive.org/download/cu31924099427845/cu31924099427845.pdf
Edward III (1327-1377)
Vol VII
(1-9) Edward III
http://www.archive.org/download/cu31924011387853/cu31924011387853.pdf
Vol VIII
(10-20) Edward III
http://www.archive.org/download/cu31924011387861/cu31924011387861.pdf
Vol IX
(21-25) Edward III
http://www.archive.org/download/cu31924011387879/cu31924011387879.pdf
Vol X Edward III Not found on-line
Vol XI
(35-38) Edward III
http://www.archive.org/download/cu31924011387895/cu31924011387895.pdf
Vol XII
(39-43) Edward III
http://www.archive.org/download/cu31924011387903/cu31924011387903.pdf
G.S. Fry (editor) (1896)
Abstracts of inquisitions for 1 Henry VII (1485) to
3 Elizabeth I (1561). First published by the London
and Middlesex Archaeological Society, and jointly
with the British Record Society, as volume 15 of the
series of the Index Library.
E. A. Fry (editor) (1908)
Abstracts of inquisitions for 19-45 Elizabeth I
(1577 to 1603).
Abstracts of Inquisitions Post Mortem, Made by
Christopher Towneley and Roger Dodsworth Vol I
Chetham Society Vol 95 - (not found on-line)
Abstracts of Inquisitions Post Mortem, Made by
Christopher Towneley and Roger Dodsworth Vol II
Chetham Society Vol 99 - Google pdf
Calendar of inquisitions post mortem and other
analogous documents preserved in the Public Record
Office.
Vol. XXV: 16-20 Henry VI (1437-1442) - Edited by Claire
Noble Calendar of inquisitions post mortem and other
analogous documents preserved in the Public Record
Office.
Vol. XXVI: 21-25 Henry VI (1442-1447) - Edited by
Matthew L. Holford
Brown, William, Yorkshire Inquisitions of the Reigns of Henry III and Edward I (Worksop, Nottingham: s.n., 1892-1906.), 1:100, Family History Library, 942.74 B4a v. 12.
Calendar of charters and rolls preserved in the Bodleian library