Maybe it is not the best place to ask that question, but I still cannot understand why we have the perfectly understandable sentence "Hon bor i det största huset" here, and in one of the previous examples we've got "Det är den sämsta bok jag vet" (bok, not boken). Could someone explain that to me?
In "hon bor i det största huset", you have a relatively simple sentence where the noun is doubly definite (det ... -et) because of the adjective.
Generally, the definite suffix (-et) can be dropped altogether if the noun is further defined by a subclause, here "[som] jag vet".
Tack så mycket för både frågan och svaret! Jag förstade det inte men jag förstår nu. (Jag tror och hoppas!)
A lingot for you both!
(I came to the comments to ask exactly that having remained confused after perusing the comments on the "Det är den sämsta bok jag vet" question.)
I would point out that this sentence is not only an adjective but a superlative. The rule is somewhat more complex, and it's not explained adequately on DL. The notes are very brief.
The standard superlative ending is -ast, which becomes -aste in the attributive (appears before the noun) definite case. In either case, there is no separate masculine form. Don't confuse this with the normal -a/-e rule Lundgren8 described.
Adjective: varm, varma (def. or plur.), varme (optional sing. m. def.)
Superlative: varmast (indef./predicative), varmaste (def./attrib.)
For irregular superlatives like störst, you add an -a for the attributive definite case, and now there is a separate masculine form. When you have a single definite masculine thing, you can use the -e ending.
Adjective: stor, stora (definite or plural), store (optional sing. m. def.)
Superlative: störst (indef./pred.), största (def./attrib.), störste (sing. m. def.)