This article was first published in May 2018.
Investing is usually a matter of judgment as well as calculation. What will do well, and what will do best over the period you envisage? Investing in books — or art of any kind — includes this element, but it also includes intrinsic interest, pleasure, taste.
A good book dealer at the top of the trade will offer advice about choices and help you to find rare volumes, but unless you have a passion for a particular field and a willingness to learn more about it, the exercise will feel rather barren. You can take a punt on anything from archaeology to adverting, Dr Jekyll to Dr Seuss. There are classic works in every field. Recently I met a farmer who collects books about pigs, right back to the 17th century.
If you enjoy a modern writer and can afford to be patient, find the finest copies you can of his or her earliest books, in dust-jackets. With the smallest print-runs, these books will be the hardest to find. A full set of Ian McEwans would be £2000-£3000 now, though the later ones are common (and he signs them on an industrial scale).