The P.H. Costello Papers consist entirely of correspondence to and from Costello, 1934-1944. The majority consists of correspondence between Costello and John Moses. The two knew each other from their days together at UND, in addition to their joint affiliation with the Democratic Party. Much of the correspondence relates to the political climate in Griggs County and the status of the Democratic Party in North Dakota, as well as the nature of politics statewide. The two were close friends, however, as they frequently discussed happenings in each other's families, as well as the health of Moses.

The collection also contains a 1936 letter from Costello to William Langer. Costello, then president of the American Pharmaceutical Association, pledged to assist the Governor-Elect with any questions he had regarding food and drug legislation and public health.

In addition, there is correspondence to and from Thomas Moodie, who was the Democratic candidate for Governor in 1934. Both Moodie and Costello express pleasure regarding levels of support for the Democratic Party in Griggs County.

Also included are several 1942 letters of recommendation on the behalf of Costello to the NABP. Finally, several letters written by Costello regarding the job applications of friends are present.

The correspondence is arranged alphabetically.