Learn Hardcore Spanish (Latin America): Toma este libro y devuélvelo a la biblioteca enseguida. - Take this book and return it to the library right away.

Toma is the informal second-person (tú) singular command form of the verb tomar (to take). In Spanish, regular -ar verbs typically form the informal imperative by removing the ending -ar and keeping only the stem—hence "toma" instead of "tomar".

What does "devuélvelo" mean, and why is it spelled that way?

Devuélvelo is the command form of "devolver" (to return) combined with the direct object pronoun lo (meaning "it," referring back to "el libro"). The accent on "devuélvelo" follows Spanish accentuation rules: when attaching an object pronoun to a verb in its command form (devuelve + lo), you often add a written accent to preserve the original stress of the verb.

Why does the sentence use "lo" instead of "la" or "los"?

Since "libro" is masculine and singular in Spanish, the direct object pronoun lo replaces it. If it were a feminine singular noun like "carta" (letter), you would use la instead, and for plural masculine objects (like "libros"), you would use los.

What does "enseguida" mean, and could I use "inmediatamente" instead?

Both enseguida and inmediatamente roughly mean "right away" or "immediately". Enseguida is slightly more colloquial and is commonly used in everyday speech in Latin America, while inmediatamente is also correct but sounds a touch more formal or direct.

Could I just say "Lleva este libro a la biblioteca" instead of "Toma este libro y devuélvelo a la biblioteca"?

You could say lleva este libro a la biblioteca ("take this book to the library"), but devuélvelo specifically implies that the book is being returned to its original place. "Llevar" just means "to take," whereas "devolver" means "to return" something.