IANAL, but to me, looks like the important part here is "the solution manual of mine contains every exercise given in the book".
Now it feels safe to assume that, without the exercises, the book would be considerably less useful as a teaching aid. In which case, the exercises are a "substantial portion" of the work.
Therefore, reproducing them and their solutions as a separate work would require reproducing a "substantial portion" of the work. It could reasonably be argued that anyone who bought the book in order to do the exercises (for a textbook, this is anyone who is studying for an exam) would be more likely to purchase your derivative work, so you would have harmed their market by publishing.
If, instead, you only gave the question numbers, and not the questions themselves, then I'd guess that no copyright issue could be argued, any more than publishing a walkthrough for a computer game.
However, it would be in the publisher's interest to change some of the questions in their next edition, and then sell their own solution book, so odds are your book would only work for a single edition.
The publisher is more likely to go to the original author and ask if they'd provide an answer book, than to accept one from you, even pre-written.
[Edit: if it is a textbook, odds are very high that there already IS a teacher's guide produced by the publisher, with all the answers. In which case, yeah, you'd be essentially competing directly against them, and they will come after you, whether or not they have a case. Defending yourself might work, but odds are their tactic will be to keep suing and appealing until you give up or run out of money. A publisher WILL have bigger and better lawyers than you, and wILL defend itself from a direct attack on its revenue like this - if they let one slide, then there'd be nothing to stop you bringing out a second, for another book, and so on.]