It may be of interest to you to read my review of the first book, The Cross-Time Engineer
The High-Tech Knight picks up seamlessly where its predecessor left off. One new narrative device that Frankowski employs is alternate narrators: this novel begins in the voice of Sir Vladimir, initially a minor character, but one who will become increasingly important.
As the novel opens, Conrad Stargard has been slowly expanding his wealth and influence as he introduces new machines and better quality of life at his adopted home, Okoitz. In bringing modern technology to medieval Poland, Conrad is setting into motion a plan that will (hopefully) allow the backwards country to defeat a Mongol invasion in 8 or 9 years time.
The High-Tech Knight contains much of the same narrative stuff as The Cross-Time Engineer: Conrad makes new machines, gains more wealth, kills a few more bad people, and has a godawful amount of sex. The major story arc which is unique to this particular book is an incident wherein Conrad and Sir Vladimir save a gross of Pruthenian children from being sold into slavery by the wicked Knights of the Cross. To modern readers, this seems only natural, but you can bet that in backwards medieval Poland, a Church-sanctioned group selling heathens as slaves is just peachy keen, and so the issue can only be resolved via a duel to the death between Conrad and and the Knights of the Cross’ champion.
I won’t tell you how it ends—though you can certain extrapolate such information from the fact that there are more books—but in fact the story arc is a minor plot point compared to the smaller strings of narrative that go into the invention of each new device. I think this is what I find so charming about these books: each chapter is almost like a short story, and while some stories are fights and some are basically sexcapades, most detail the vagaries of invention. Conrad teaches algebra, but first he creates a base-twelve numbering system. Conrad builds a coke oven, but first has to drain a coal mine so he can dig for clay. It goes on. Maybe it’s just me, but that’s the sort of thing I like.