All books cited in this guide are available in either electronic or printed form, with some in both options. To connect to an e-book, click the citation link, and then click on the link under Online in the catalog record, which leads to the book on a publisher or vendor website. In most cases, access is available only to authorized Princeton users. To locate printed books in the Mendel collection, click on "Where to find it" beneath "Location" in the catalog record. E-books sometimes bear dates later than the print but do not necessarily offer corrected or revised texts of the print version.