It depends on the gender of the object and the case. Nominative Case MASC, NEUT, FEM: "Meiner, meins, meine" or "deiner, deins, deine" if it doesn't come before a noun. And if it comes before a noun it will be: "mein, mein, meine" or "dein, dein, deine" https://deutsch.lingolia.com/en/grammar/pronouns/possessive-pronouns

Couldn't you also just say "mein" and "dein" without the "-s" in colloquial speech?

Kann man auch "Das Buch ist meins." sagen denn?

Meins and deins. For example: Das ist mein Buch (that is my Book). You then omit the noun (Buch): Das ist meins (That's mine). Same with yours: Das ist dein Buch (This is your book) and Das ist deins (That is yours)