Well done! Keep at it... The only thing that might have annoyed me was the fact that the agent was "mostly non-fiction". So, how much "fiction" has he represented? 10%? 1%? None? Doesn't mean he doesn't know the genre, but...
This is so true. I have a writer friend in the same position - signed with a high profile agent one of who's clients had a huge debut (with very large advance) so assumed things would proceed nicely, only to find the agent doesn't even read the manuscripts she sends her. Needless to say, the friend is looking for new representation.
If literary agents work like theatrical agents do, there's a trade off.
'Well established' will be less able to/likely to concentrate on you as an individual since the bulk of their income (and therefore attention) will be focused on their already-established, successful, lucrative clients. But they will have lots of ready-to-go contacts and relationships with the people who ultimately will employ your services (producers and casting directors / publishers) and therefore are more likely to have access to a wider spread of the available opportunities.
'Young and hungry' are more likely to concentrate their attention and efforts on you since they'll be more dependent on your success to improve their own income. The downside, though, is that they may not have the same industry 'clout' or ready-made access to the bigger players in terms of placing you. They (an you) may not even see a lot of the opportunities that are out there, since the established agencies will have done the deals over a lunch somewhere before the smaller agencies even know about them (if, indeed, they ever do).
The ideal (at least in the acting business) was always to try and get taken on by someone who used to be with a bigger, more established agency (and thus has the contacts) but has recently left to start up on their own (and therefore has a smaller and developing client list). These 'unicorns' embody the best of both worlds... and are consequently a) as rare as dragon eggs, and b) hugely over-subscribed!
Glad to hear that the feedback you got was useful. Regarding agents it's worth registering for free with The Bookseller. A free account gives you limited access but you should be able to see most of the debut book deal announcements which is a good way to see which agents are selling debuts in your genre and target them for submissions.
Also when you are at the bookstore if you see any debuts in your genre check the acknowledgements section as author always thanks their agent in there. You can also google the author's name + literary agents to find out who represents them.
It's worth keeping in mind, with regards checking out new debut's agents - especially at publication - that there's a large gap between the agent taking the author on, and the book being published. The agent's circumstances may have changed considerably in that timeframe.
New deals will be be a more reliable source of present circumstances.