What does the phrase "none too dissimilar" mean?

Solution 1:

None too means not very, so the sentence means that the book and the movie were not very different.

Solution 2:

None too dissimilar means practically nothing.

All three words are negatives, i.e they all generate NPIs, and the relations between them are hardly spelled out.

None is a negative existential quantifier meaning not one or not any. Logically ¬(∃x).

Too is a quantificational adverb with complex syntax. If something is too P then it is P to a degree that causes some negative outcome. Logically (∃x) (Degree(P(x)) ∧ (Degree(P(x)) ⊃ ¬S).

Dissimilar is a symmetric negative predicate with two arguments -- either of which can be subject if the other is object, which is not the norm. The overt negative states that the degree of similarity is low; I won't bother with the logic here; the point is, it's complicated.

Hence these can be combined in any number of different ways, and it's simply not clear what this speaker or writer meant by the phrase. This process of overnegation seems destined to go on forever, as Larry Horn has pointed out.

Solution 3:

I think the OP's sentence is OK as long as it's meant with a degree of sarcasm i.e. if the expectation is that the book and movie were supposed to be different, then their description as "none too dissimilar" is making the point that they were indeed similar.