How do i build a bookcase door (fot a secret room)?

Just use any kind of wood, pine would do it the picture of the handle is what you are referring to. P.S. if this is you hiding space put a tiny knob in the back and so you can hide it by covering it with books or something. The large handle is too obvious. It screams open me and does not look very secretive.

Hardwoods like cherry and maple are much more resilient than mdf or pine. Unfortunately, to do rhis yourself, it is more economical to go to a lumber yard rather than lowes or home depot where the finished hardwoods are more expensive. If this is a moisture prone area, it is best to paint a poly coating over door

3/4” A/B plywood. Spans cannot be more than 3’.

Now I see from the picture what you are referring to. The only picture I saw was the one posted with the drawer pull. Builder grade plywood would be your best bet. I just did a pantry with very heavy kitchen appliances, canned goods etc. It has a beautiful finish to it as well. I see what you are calling hidden now. Consider purchasing white hinges for replacement. If you paint them yourself the paint will eventually flake off. Right now they are a dead give away that you have something "hidden", they stand out too much.

Things to consider:

•Any wood product swells and contracts with the amount of moisture in the air. You should store any wood you purchase in your house, not in your garage or storage shed, for at least a week before you start. You can take it outside to cut it, but project materials should be stored inside.

•Plywood is best for the back of the case. It's right-angled plies hold better against the diagonal forces from the hinged side to the unhinged side. Use both screws and glue to securely fasten together all parts including shelves. You can have adjustable shelves as long as at least one shelf at 1/3 to 1/2 of the height is permanently secured for greater structural integrity.

•Don't underestimate the weight of a bookcase and its contents. Plywood weighs roughly 40.6 lbs for a 1/2" thick sheet, while MDF weighs 66.7 lbs. Each 6x9 hardback book with 200 pages weighs about 1 lb. Hinges should be sized to a maximum loaded weight. Most hidden doors use pins at the top and bottom and the door swivels on those, though there are heavy duty hidden hinges that are mortised into the framing.

•Existing doorways may not be level, square or plumb. Framing lumber is not always perfect, so doors and their frames "float" between the studs. If you attached your bookcase door to the existing hinges the wider bookcase will emphasize any out of level, plumb or square that wasn't apparent with the thinner door.

https://www.iliketomakestuff.com/secret-door-bookcase/

https://prepforshtf.com/build-secret-bookcase-door/

https://diyprojects.ideas2live4.com/turn-a-bookcase-into-a-secret-door/