Crossword Genius

According to the book that's reserved (6)

Ross

I believe the answer is:

formal

I'm a little stuck... Click here to teach me more about this clue!

'reserved' is the definition.
(similar in meaning)

'according to the book' is the wordplay.
I cannot quite understand how this works, but
'to' could be 'for' and 'for' is found in the answer.
The remaining letters 'mal' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'that's' is the link.

Can you help me to learn more?

(Other definitions for formal that I've seen before include "It's required by rule" , "Conforming to accepted rules or customs" , "Befitting a person in authority" , "Conforming to accepted customs, say dress" , "Ceremonious or official" .)

I've seen this clue in the Evening Standard.
Want a hint initially instead of a full solution? Install my app