Why is "angalia kitabu" wrong? Why does it have to be the plural form?
It's an error; report it if it happens again.
Is the "eni" suffix supposed to indicate "at the"? Because ive always understood it to change the command angalia/look to the equivalent of angalieni/look y'all.
-ni is the locative suffix, but it can also be to distinguish you(singular) from you(plural) (ie, karibu(welcome) ---> karibuni(welcome to multiple people))
Here it's being used in imperative form, and it has the plural distinguisher -ni.
In this case, the "at" is implied by the verb kuangalia. In English, "look" is an intransitive verb, so if you want to specify an object, you have to use the preposition "at", but in Swahili, angalia is a transitive verb, so the "at" is unnecessary.