This is the link to the history of the Topeka chapter of Delta Sigma Theta's (DST) page.
Nick Chiles' daughter Thelma was one of the founding members of the DST chapter at Michigan. I haven't found a lot about Thela and the DST, but I do know that Thelma became a librarian, and the DST promoted education and in specific libraries. One of their early initiates was mobile libraries. Just looking at the dates I do not think that the DST chapter in Topeka was responsible for the bookmobiles here (here is TSPL's link to the founding of the bookmobileshttps://tscpl.org/history/branches-bookmobiles) but the timing is interesting, this was obviously a going concern at the time. Because every good post has to have a picture, here is the earliest picture (from TSCPL archives--gotta love it, it looks like an Airstream--those Topeka librarians in the 40s were budding hipsters) that I could find of the bookmobiles in Topeka. For more information about bookmobiles (and great pictures) here is a website https://ebookfriendly.com/retro-photos-libraries-wheels/.