I will try and keep things as simple as I can.
I will assume that although you want to self-publish your book using a service like Blurb
(which is an American company, by the way), you yourself will be in Italy. The place of publication will then be Italy, irrespective of the fact that Blurb is a US company. It doesn't matter what language the book is in, but if it is to be Italian, then this strengthens the fact that publication is in Italy. This will mean that the starting point should be Italian law. If you would like to read what the Italian law says (in Italian) on the subject of copyright then you can find it in this link
. However it really isn't necessary to read the law in order to understand a few basic concepts.
Perhaps I should start by saying that you may make a complete copy of the whole of the forum discussion by saving it on your computer so that you can access it even if you are not on the forum at the time. This is just for the purpose of your own private study, and the legal permission for this is derived from Article 69 of the law quoted above.
Next I need to make it clear that you own the copyright in everything that you have written on the forum, so there is no problem with you publishing those postings in your book. However for the same reason, the other people on the forum will own the copyright in what they have written and this means that you may not reproduce their words verbatim (by which I mean using exactly the same words that they wrote), without getting their permission. However your postings will make no sense if you can't include the other postings. You could ask your fellow posters for their permission. That may or may not be granted. Or you may be able to quote small pieces of their postings if this is sufficient to give the context of your replies. The legal exception for this purpose is contained in Article 70 of the Law no. 633 of 22 April 1941
Art. 70. 1. Il riassunto, la citazione o la riproduzione di brani o di parti di opera e la loro comunicazione al pubblico sono liberi se effettuati per uso di critica o di discussione, nei limiti giustificati da tali fini e purche' non costituiscano concorrenza all'utilizzazione economica dell'opera; se effettuati a fini di insegnamento o di ricerca scientifica l'utilizzo deve inoltre avvenire per finalita' illustrative e per fini non commerciali. ((1-bis. E' consentita la libera pubblicazione attraverso la rete internet, a titolo gratuito, di immagini e musiche a bassa risoluzione o degradate, per uso didattico o scientifico e solo nel caso in cui tale utilizzo non sia a scopo di lucro. Con decreto del Ministro per i beni e le attivita' culturali, sentiti il Ministro della pubblica istruzione e il Ministro dell'universita' e della ricerca, previo parere delle Commissioni parlamentari competenti, sono definiti i limiti all'uso didattico o scientifico di cui al presente comma)). 2. Nelle antologie ad uso scolastico la riproduzione non puo' superare la misura determinata dal regolamento, il quale fissa la modalita' per la determinazione dell'equo compenso. 3. Il riassunto, la citazione o la riproduzione debbono essere sempre accompagnati dalla menzione del titolo dell'opera, dei nomi dell'autore, dell'editore e, se si tratti di traduzione, del traduttore, qualora tali indicazioni figurino sull'opera riprodotta.
However as you can see this exception cannot be used where the purpose is commercial, and so unless you want to give your books away for free, this exception is not going to be much help to you. That leaves you with one other option which is to paraphrase the contributions of the other posters in a way that does not use their exact words, but still conveys the same meaning within the overall discussion.
Clearly the best option from your point of view is the first one, that is to say, getting permission from the other contributors. It may be that some contributors will agree and others will not and so you may need to combine the direct quotations which have permission together with paraphrasing for those who refuse permission.
I hope this is helpful. If you would like me to expand on anything, please feel free to come back with more questions.