The so-called Sienese sketchbook of the famous architect and engineer Giuliano da Sangallo was originally in the library of Sienese scholar Giovanni Antonio Pecci. The librarian Giuseppe Ciaccheri, a committed and passionate collector who enriched the Biblioteca comunale degli Intronati di Siena with works of art of outstanding quality, acquired it in 1784. Together with the Codice Barberiniano in the Biblioteca Apostolica Vaticana, the sketchbook bears witness to the architect's prolific production of drawings and is a valuable source of knowledge about his work. The small format and the style of the drawings indicate that the book was a personal study and work tool. Extremely varied, it includes sketches, mostly with an architectural subject and often accompanied by measurements and technical notes, ideas for projects (such as the one for the dome of the Basilica of Saint Mary in Loreto), drawings of machine and artillery pieces, and copies of classical sculptures. It also has studies of monuments observed in the course of Giuliano’s travels in Italy and France (among which are triumphal arches and the Coliseum), copies of reliefs, drawings of decorations (panoplies, grotesques, frames), and even some sketches of capital letters from public inscriptions. Giuliano appears to have had a significant interest in medieval architecture, as attested, for instance, by the sketches of a number of buildings in Pisa and of the Tower of the Asinelli in Bologna. The leaf with the elevation of the Piccolomini Chapel in the Cathedral of Siena dates back to one of his stays in Siena. The sketchbook also contains plans for a building for the Sapienza of Siena, which, thanks to an inscription, can be linked to Cardinal Francesco Piccolomini's decision, circa 1492–93, to build a new structure in addition to the existing Sapienza (located since 1415 in the space previously occupied by the Misericordia Hospital and the present-day site of the Biblioteca comunale). The nature of these sketches is as yet unresolved: some scholars see them as drawings for the renovation of the existing building; others think they were the designs for a new building. A drawing in Pecci’s hand of a cross-section of the Sapienza building, based on Giuliano’s original plans, is inserted in the codex. The sketchbook provides a window into Giuliano’s deep and multi-layered artistic culture, as well as bears witness to his intense study of classical models as an integral part of his work. The drawings probably date to the architect's later years, from the 1490s to 1516. At folios 1v–2r are some glue-making recipes in a 16th-century hand not ascribable to Giuliano.