In hopes of balancing things across the Atlantic, the catalogue of the University of California (based at http://www.cdlib.org) is searchable for periodical and non-periodical items and results give an indication of location. I have listed non-recording items found under Foulds' name.
Essays in the modes (Essais sur des modes)
Publisher: Neuchâtel: Les Editions du vent qui passe
Vol.1. Exotic (Exotique); Ingenuous (Ingénu); Introversive (Introversive); Military (Militaire); Strophic (Strophique); Prismic (Prismique); Eroico.
For piano.; includes a commentary and biographical sketch of the composer in French (8,  leaves) inserted.
Holdings: UCB-Music-M25.F65 E8 1990z
Music to-day; its heritage from the past, and legacy to the future.
Publisher: London, I. Nicholson and Watson, limited, 1934.
Holdings: UCSB-Arts Lib-ML197.F62 M8
Author: Macdonald, Malcolm
John Foulds and his music : an introduction ; with a catalog of the composer's works and a brief miscellany of his writings.
Publisher: White Plains, New York : Pro/Am Music Resources, 1989
xii, 150 pages
[Nb. This is the updated and revised edition which expands on aspects of biography and discography, and so some degree simplifies his work catalogue]
Holdings: UCB-Music-ML410.F785 M23 1989
UCD-Shields-ML410.F79 M3 1989
UCI-Main Lib-ML410.F785 M2 1989
UCR-Rivera-ML410.F675 M2 1989
UCSB-Arts Lib-ML410.F63 M19 1989
UCSD-Music-ML410.F785 M22 1989
Variazioni ed improvvisati su un tema originale (Op.4) [for piano solo]
Publisher: London: Novello, c1905.
British 20th-century piano music. Volume 3 / selected and introduced by
Publisher: [London?]: Thames Publishing, c2000.
45 pages of music; 30 cm.
Contents include 'Introversive' from Essays in the Modes by Foulds (and programme notes)
Holdings: UCB-Music-M21 .B7583 2000
UCLA-Music-M21 .B7773 2000
Some examples of locations:
UCSB = UC Santa Barbara (http://www.library.ucsb.edu)
UCSD = UC San Diego (http://orpheus.ucsd.edu/library)
UCB = UC Berkeley (http://www.lib.berkeley.edu)
UCR = UC Riverside (http://library.ucr.edu)
UCI = UC Irvine (http://www.lib.uci.edu)