The current edition of Harrison's is number 19. Because of how thick the book is, my understanding is that the hardback can be put into two volumes, but the softback comes in three volumes because the spine would break more easily if it was thicker. So if it's for a long-term addition to your library, go for the hardback. If it's for short term used in medical school or just passing your boards than the three volume format is obviously more portable. In medicine NEVER buy the previous edition. Leading edge medical information usually takes 5 to 7 years to make it to the textbook phase. It's not worth spending 200 to 300 hours reading a textbook that's more than seven or nine years out of date to save $100 on the price of a textbook. If you were nonmedical and just want to get some general information about Medicine and disease then an older version is fine, but if you are a practicing physician either now or in the future you should always use the latest textbook to get the newest and best information .