Hardcover? Or paperback?
Illustrations? Or just text?
First editions? Or you don’t care?
Signed by the author? Or not?
If I were to collect books for their monetary value, I would say that yes, hardcover, illustrated, first editions, signed would be the way to go. (Except didn't I see somewhere that signed can sometimes weaken the value? Or maybe that's if it's personalized? So if you care about that stuff, you'd get it signed, but not have them write it to you?)
But, since I don't care about that stuff, I pretty much have a mixture of all of the above.
Hardcovers are nice, but harder to read. And once you start a series in hardcover, you feel you need to keep them all the same, right? Or maybe you started getting them in paperback, then it's a problem when the last of the series comes out and you want to buy it in hardcover? These are the stresses I have!
Illustrated versions of novels are cool, but I don't really have any.
First editions are cool I think only if you are a big-wig collector. I doubt any of my first editions will be worth something someday. Maybe in 500 years or so. I don't have any first editions from years past, though if I did get my hands on something like that, I would think it pretty cool.
Signed copies are cool. I love my signed copies, whether they be first editions or not, hardback or not. And who cares if personalized brings down it's monetary worth or not, it's worth more to me if it's personalized!