Well, when you really get into it, you realise that the name Jeet Kun Do is used to show a lineage, not a specific style. Different JKD groups can be worlds apart.
But that's not the spirit of the question...
You can learn a lot from books and videos, In fact without any instructor ever you can still learn a lot.
But, and this is a big but, you do need people to train against. Same as you can get quite good at basketball playing on the street with friends and no cach, you can get good at fighting too. But you need opponents, and will be limited by their abilities as well.
It's also going to be a lot slower, if you have a good coach they will pick up mistakes right away, and correct them right away. Starting from scratch with other new guys will not get that, it might take months or years for them to figure out how to capitalize on the mistake, and equally long to figure out the way to fix it.