We describe six mathematical books digitized and available online in the Virtual Library of the National Digitization Centre (Virtual Library, http://elib.matf.bg.ac.yu:8080/virlib/). The first two books are the oldest printed mathematical textbooks, Aritmetika (arithmetic) by Vasilije Damjanović printed in 1767 and Cislenica (old form in Serbian of the word arithmetic) by Jovan Došenović printed in 1809. These old books are elementary from the mathematical point of view, but, certainly, of interest to the history of the Serbian people, especially to studies of the Serbian language because they were written before Vuk Karadžić’s reform of language and orthography. The other four books are gymnasium textbooks written by Rista Karljiković: Trigonometrija, Algebra, Analiticka geometrija and Geometrija (Trigonometry, Algebra, Analytic Geometry and Geometry). These excellent textbooks were used in Serbian gymnasiums and other secondary schools between the two World Wars (1922–1941). The digitization of these books is part of the project of forming electronic archives containing Serbian mathematical books printed in the past. In the article other reasons why these books are selected to be included in the Virtual Library are also given. The contents of these books are briefly presented together with some interesting points.