Read this somewhere in reddit. Is this a fallacy?

I don't think this is a fallacy, but rather false premises.

It's false that every book has gems of wisdom, and Hitler's book is proof of that.

I see this as a wisecrack - something a late night host would say to get a laugh while making a point. As far as weak analogy goes, this is subjective. One would have to argue why it is more unlike than like. I would be on the side for it being more like. The point being made, as I see it, is that there are far better sources of wisdom available that don't include as much nonsense or even bad or harmful advice/commentary. Of course, one can disagree with the implied claim that the books used in the example fit this category.

Depending on what book X is it might be considered a weak analogy 

Person B needs to expand on what they mean, though. Why is book X bad, in their opinion?

This is (as earlier mentioned), a terrible analogy.

An Argument By Analogy has the form:

A has attributes a, b, c etc. and z (or possibly y and w as well)

B has attributes a, b, and c etc. 

Therefore, B probably has z etc. also.

The elements that are compared are called analogues.  a, b and c in the above form are called "primary analogues."     x is called the "secondary analogue."  

In determining whether an argument is a good analogy rather than committing the fallacy of false or weak analogy the following factors are relevant:

1.  How relevant are the similarities between the primary and secondary analogues to each other.

2. The number of similarities between the primary and secondary analogues  (how many a,b,c vs x,y,z etc.  are similar)

3. The number of relevant dissimilarities between the primary and secondary analogues.

4.  The number of primary analogues.

5.  Diversity among the primary analogues.  

6.  How specific the conclusion is.  The more specific, the weaker the argument by analogy is.

-----I think it's clear that an analogy between books and a pile of feces doesn't pass the test based on the above factors.  

The above is paraphrased from Patrick Hurley, A Concise Introduction To Logic