Knight of the Sacred Seal is very nice, but I'd hold off on that until you can start binding Zceryll. If bloodlines are allowed I would recommend those to give faster progression through your Effective Binder Level. Being able to use Summon Monster at will, 1/5 rounds, is incredibly powerful as my DM found out again last night.
Our Party got ambushed by a group of 20 legion devils, 4 metal covered devils (had a discordant aura that disrupted spell casting don't know their names), and 4 Orthos devils (big spear wielders that when killed exploded into a burst of maggots). My DM swears this was supposed to be a slightly tougher then standard CR 14 fight I don't think he took all of our party's handicaps into account. We'd already faced 4 or 5 even CR fights that day, as well as had a large group of commoners, 30 or so, we rescued in tow down in a mine looking for an exit to get them out. We hadn't had time since our last encounter to fully heal back up and were running low on healing outside my summons. I felt the group of devils coming with enough time to send my currently summoned earth elemental to charge into their ranks to buy us some time, 12 rounds, while we had the commoners hide back in the mine tunnels and we tried to prepare and ambush of our own.
Poor tactics in the opening round by our archer lead to him getting surrounded by the legion devils right off the bat. Our Swift Hunter Archer, Sorcerer, and Monk/Kensai all got smacked hard in the first two rounds of combat and ended up having to flee or teleport away to avoid getting swarmed under by the legion devils.
My Binder Jacob had no such options being weighed down by Full plate and having Naberious, Orthos, and Zceryll bound at the time. Instead I backed up against a wall and let loose with my breath weapon, suggestions, and summons while hoping that my AC 30 and displacement, 50% miss chance, kept me alive long enough to get my summons off. The Celestial Triceratops with its dc 28 trample ability proved to be particularly effect against the group that was surrounding my character. After a long fight and turning one of the Orthos devils against the others Jacob stood wounded, 25 hp remaining, but alive.
I definitely need to get him some different DR as DR 10/magic has proven mostly useless against appropriately difficult CRs.