The choice of best option between buying new book or second hand book or opting for a book rental is balance of time you need it for and book cost. A simple rule of thumb is if a book is needed for more than a year purchase works out better than leasing the book. If your need of the book is time limited (like for exam, only reference for short period) like a semester nothing can beat rentals. As far as purchasing second hand book is concerned books that change very frequently like GATE books, ESE books, GRE, GMAT Books, SSE Bank PO which have newest year based updates and you cant have the access of such material separately you should go for a new book purchase. However if the book like IIT JEE book or NCERT book which has most of the content and newest question papers are available else where second hand books save you most money. Most of the books wouldnt have any big changes and remain same for years to come. Its a no brainer to opt for a book rental or a used book purchase in such scenarios.