Young Flandry

The Young Flandry Trilogy systematically transforms Dominic Flandry into the character that he had already been in the earlier written Flandry series. (One novel, Casino Royale, transformed James Bond into the character that he would be for the rest of that series.) 

The second Young Flandry novel, A Circus Of Hells, adds several layers to Flandry's character:

now training for Intelligence work, he learns surveillance on some routine solo scouting missions; 

he learns from several mistakes - check everything, let your enemy underestimate you, don't let them know how much you know but also conceal your ignorance;

accepting a large bribe enables him to advance his career - nest-feathering, special equipment, social contacts, unbudgeted pleasures;

helping to enrich a local gang boss ensures that that boss will pressure the Empire not to abandon Irumclaw - thus, Flandry combines personal advancement with defending the Empire;

he spends a lot more time in the company of Merseians, thus improving his Eriau and increasing his knowledge of the enemy;

he is instrumental in founding the joint Terran-Merseian scientific base on Talwin - or, to re-express this in political terms, he ensures that Talwin is of no further use as an advance base for Merseian Intelligence but instead becomes a discrete site for confidential inter-imperium talks, including later an interesting conversation between Flandry himself and his opposite number, Aycharaych;

he demonstrates to the reader, and possibly even to himself, that he does not understand women;

he is placed under a geas that he will never get the woman he really wants - this will have catastrophic personal repercussions in the following novel, The Rebel Worlds, and again later in A Knight Of Ghosts And Shadows.

And there is much more in the novel about the other characters, Irumclaw, Wayland, Talwin, the Empire and the Merseians. Poul Anderson conveyed a lot of condensed information in many short novels and also wrote several longer ones.