In order to locate material on your topic at the Princeton University Library, you will have to search in the Library Catalog. Keyword searches are fine, but one should also know how to search using Subject (browse) for a specific garden, site, topic, or theme, e.g. Central Park (New York, N.Y.) along with its many sub-headings; a specific designer, e.g. Olmsted, Frederick Law, 1822-1903 (search by LAST NAME first); or more generally:
Subject heading examples (most followed by various sub-headings; sub-divided geographically, etc.):
GARDENS UNITED STATES
GARDENS DESIGN
GARDENS, ENGLISH (GARDENS, FRENCH, etc.)
LANDSCAPE ARCHITECTURE
LANDSCAPE DESIGN
LANDSCAPE GARDENING
LANDSCAPES
HORTICULTURE
BOTANY
BOTANICAL ARTISTS etc. . . . .