Depends what you want.
I recommend:
- Evangelion : Artwork and Illustration Collection :
No idea how old it is and if you can find it. First half is full page Sadamoto paintings and characters arts, the ones that made it to be the manga covers. These are the most beautiful and iconic Eva artworks from the late 90s. Second half is detailed black pen of every characters, costumes, robots, vehicules, rooms, accesories, gizmo, building, everything. It's a goddam treasure for the anime fans. But its not the usual " Groundworks " you can find for animation. This is not the story board with early animation sketches (Eva has a lot of these books, I wont cover them here as Im not so found of them)
- Die Sterne :
This one is rather easy to find, its a collection of official works made for the show and all the merch around it. Its not always made by the original artists but that the point : it will be characters art for coffee commercial, travel brands, fashion brands, OST covers, anniversary illustration... it's like an official fan art book for all the licensed Eva art. It's amazing, I love this one for the variety it shows and how you can find the characters we love being dressed casual, hiking, biking, etc... It has a few full pages of original Sadamoto art at the end, gorgeous but ultimately a bit redundant with the early pages of the book I mentioned on top.
- Evangelion Illustration 2007 - 2017 :
This is Die Sterne, but for the new movies called " Rebuild ". If you enjoyed the new movies, go for it. (we are still waiting for the 4th and final Rebuild movie)
- Der Mond :
This is a Sadamoto (evangelion lead chara designer) illustration book, covering Evangelion but also his earlier work at gainax : Nadia of the Blue water, Wings of honneamise etc... This is short but beautiful if you really like Sadamoto soft colors and painting.
Udon is also bringing this to the US (or is it already available ?) :
And I have no idea what is it but I suspect it might be the detailed character and vehicules and building plans I mentioned in the first book so this might be good.