I don't sense any difference in meaning between a. and b. Both imply the existence of (at least) three books - one that is under discussion, and two previous ones. Without further clarification, I would in both cases assume that it referred to the two most recent books before the one being discussed. Because 'previous' implies something to be previous to, so I would not simply talk about 'The author's previous two books', without a frame of reference. If you just want the two most recent, you could say "His latest two books" (or "His two latest books" - both are fine).
Example: given an author who had written five books, conveniently titled (in order of publication) A through E:
If someone said "Book D is better than the previous two.", I would assume they meant that D was better than B or C.
If they said "The latest book is better than his previous two", I would take them to mean that E was better than either C or D.
In order to refer to different previous books, they would need to specify. If they said "The latest book is longer than two previous ones.", I would ask which two, because it could be any two of A, B, C, and D.
If they said "His latest two books are excellent." I would know they meant D and E.
Because 'latest' is implied, I would generally not say "His two previous latest books" in any order, because it's redundant.
Side note: Most style guides recommend spelling out numbers less than 11: 'Two books', 'Ten books', '24 books'.