FYI, I'm in the process of finishing up my third game with the summoner mod installed.
In the first one, I got two of the "Book" techs and saw that they unlocked the corresponding summoner units, so all was happy there, although I never did get around to building any of them.
In the second, I didn't get any "Book" techs, which annoyed me, so I installed the "No more raodom book of power in magic tech"[sic] mod from Nexus, which replaces the master tech file with one that sets all for "Book" techs to 100%.
In the third game, I have all four "Book" techs, but they're not unlocking the summoner units. I have received a summoner unit as a quest reward, however, so I know your mod is still there and active.
Anyhow, that's a potential incompatibility that you may want to look into. Also, would you consider an option in the mod to make the "Book" techs 100%? (I fully understand if you don't want to deal with any more variations of the mod than necessary, but it would be nice.)
As to feedback on the summoner units themselves, they seem to work properly. The one thing I'd like to see added is some indication of the summoned elemental's level (and how it's determined, if not a fixed level) in the tooltip for the summon powers.