If you read German there are some very good books available (let me know
if you're interested).  I haven't read or researched Italian books but
I imagine there are a fair number that could be good.  Nono seems more
accepted in Germany than Italy.  There's almost nothing in English other
than two volumes devoted to Nono in the journal "Contemporary Music
Review" (Vol. 18, 1 and 2).

I prefer his late work which is no longer preoccupied with serial
deployments (e.g., Das atmende Klarsein, Fragmente-Stille, Quando
Stanno Morendo, la lontananza nostalgica, and Prometeo).  His later
work (along with Scelsi) has had a significant influence on various
European "minimalisms" (Sciarrino and the French Spectralists in
particular).

I don't know which of the early works would be considered rigorously
serial but you might want to check out Variazioni canoniche (variations
on the series from Schoenberg, op. 41), La fabbrica illuminata, Contrappunto
dialettico alla mente, Varianti.

An inexpensive way to get a chronological cross-section of Nono is col
legno (WWE 1CD 20505) - Orchestral Works and Chamber Music.  It is a
mid-priced disc.  There's also DG 471 362 of the Pollini edition, which
includes "Como una ola de fuerza y luz" and "...sofferte onde serene..."
To get "Das atmende Klarsein" I like Col legno 31871 but there is also
a good performance that does not have some of the live "white noise" on
Hannsler 93.022.  The Arditti Quartet have done all(?) the string chamber
works on three volumes for Disque Montaigne (782047, 782004, and?).
These, I think, have been re-released as mid-priced discs.  Prometeo
brings everything together for me and is available as an import from
EMI.

Enjoy your exploration,
Bob