The expression non omnino is not1 rare in literature, but in most occurrences both interpretations make sense.
I found some examples where only one of "not entirely" and "not at all" makes sense.
Augustinus in Confessiones 1.2.2 writes "non ergo essem, deus meus, non omnino essem, nisi esses in me".
Judging by context, he means that he would not exist at all if God did not exist.
Here non omnino means "not at all".
Cicero in Brutus 110 writes "his quidem non omnino ingenium, sed oratorium ingenium defuit".
He appears to mean "they were not missing all talent, but only orator's talent".
Here non omnino means "not completely".
Plautus in Asinaria 233 writes "Non omnino iam perii, est relicuom quo peream magis".
Here it seems to make more sense that non omnino means "not completely".
Tertullianus in Apologeticum 47.2 writes "Quis poetarum, quis sophistarum, qui non omnino de prophetarum fonte potaverit?"
Here non omnino means "not at all", since "not completely" makes no sense.
Judging by these examples, non omnino can mean either "not at all" or "not completely".
The meaning may have changed between different authors and eras, but in general it doesn't seem to clearly refer to either possible translation.
(I must say I am not1 satisfied with this observation.)
Therefore, if you want to make your Latin book review unambiguous, I suggest avoiding non omnino in both cases.
I might write "I didn't like the book at all" as totus liber mihi displicuit and "I didn't entirely like the book" as liber mihi haud penitus placuit.
I would be happy if someone could provide more illuminating examples.
1 You can read this as "not entirely" or "not at all".