The Practice Effect

By David Brin

Dennis Nuel was the genius behind the zievatron, a machine that opened a portal to an alternate universe, but he got pushed out of the research project and sidelined. So he was thrilled to be offered a chance to rejoin the team. But there was a catch...he would be obliged to enter the zievatron and allow himself to be transported to an unknown world, a world from which he may never be able to return.
When he exited the zievatron and onto the strange planet, he immediately discovered why the return mechanism had stopped functioning. It had been hacked to pieces. Before much longer Dennis discovered that the locals were humans who even spoke English. But their technology was at a stone age level and they regarded Dennis as a kind of wizard and he soon found himself in thrall to the local warlord who wanted him to create powerful weapons like the gun they confiscated from Dennis.

This was a fun story. Dennis soon figures out that the physics of this strange place are very different and quickly manages to use it to his advantage, winning his freedom from the tyrant and rescuing the beautiful princess in the process. He and his friends, including a strange little animal, a robot, the princess and a thief, have lots of adventures and Dennis uses his superior knowledge to create devices, that, with the strange local physics, enable them to rescue themselves from peril from the bad guys who are anxious to capture their escaped wizard. It was quite an enjoyable read.