Book them (ticket) or reserve a seat on them (traveling with Interrail / Eurail) ?

too early. the usual way. TrenItalia feels it has no reason to allow their patrons to book that early-gives only trouble if changes need to be made.

As a rough rule you can expect service between the major cities to be about same as same weekday this week-give or take an odd minute.

The real hi-speed network-just use google to find a wiki item on it-is only in the north and T-shaped-the horizontal branch of T is still under construction.

And its ridiculous to think that one has to fear for ´full trains that early.

as has been said so many times-but apparently we have to tell this each new passholder anew: do NOT rely on app to much-find seewulfs page about on howtodo REServ yourself for less-this means for Tr-It via oebb.at

Maybe I’ve missed the obvious, but I can book a reservation only on another site other than eurail???  The other sites I looked at it appears you have to actually purchase the ticket also.  We already have our eurail pass.   

The advice from the experienced travellers in the community is to use other ways to make seat reservations than the Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

Eurail changes an extra fee of 2 EUR per ticket. Most other places where you can make seat reservations have no such fee.

I must be in the remedial group because looking on multiple train company websites I don’t see an option for reservations only, even if I am looking for a train in July.  I can only find where to buy tickets.  

There are different ways for each train company how you make only a seat reservation. In the guide that is explained. 

If you give your travel details here you will get advice how to do.

On the Italiarail.com website I am trying to figure out how to make reservations only for these dates:

17 July Naples to Roma 10:40 - 12:00

20 July Roma - Firenze 9:50 - 11:27

24 July Firenze - Venezia St Lucia 10:40 - 12:00

27 July Venezia - Milano 10:48 - 13:15

All 1st class reservations. 

Italiarail.com stopped to sell reservations online (same as Trenitalia )

You can now reserve Italian trains on oebb.at By selecting Eurail/Interrail as discount and then on Single / Dayticket (Do NOT use the Seat only option when reserving italian trains on oebb.at :)) Here a short pictured manual