I believe this story was published in the late 1950s or early 1960s; it may have started as a short story and then been turned into a novel.
The basic plot was that the protagonist had created a tiny computer ("Manche") that approximated the capabilities of today's netbook, plus a degree of sentience; as the story progressed, he and Manche were fighting for Truth & Justice, etc., but were bested by the villain(ess, I think), who then stole Manche. Our hero then built a better/faster/smaller version, named Dimanche or Diamanche, and after a suitable battle, saves the day.
I don't recall anything else about the story -- not even, really, why I enjoyed it so much . . . but it was one of the pieces that spurred my love of electronics, computers, and the hybrid human/computer knowledge-retrieval made possible by sites like Stack Exchange. So it's kind of amusing that Google has been so little help finding this story! :*)