cookbooks

Both!

Both are terrific, but Plenty has been out long enough that you might be able to find it at a local library and read it without purchasing.

Jerusalem is spectacular to look at and I've been cooking my through the recipes which are really good

You can't go wrong with either. Jerusalem tells more of a narrative, and has more variety, whereas Plenty is entirely vegetable-focussed.

My best answer is both :) Ottolenghi writes a really, really good recipe and sets you up for success.

Jerusalem - both books are outstanding, yet, since you're asking to choose between the two, I like the fact that Jerusalem is not exclusively vegetarian and the recipes are all accessible for the home cook.