From Walter Isaacson's Einstein (page 116 of the linked pdf) :
Einstein’s greatest intellectual stimulation came from a poor student who dined with his family once a week. It was an old Jewish custom to take in a needy religious scholar to share the Sabbath meal; the Einsteins modified the tradition by hosting instead a medical student on Thursdays. His name was Max Talmud, and he began his weekly visits when he was 21 and Einstein was 10. .....Talmud brought Einstein science books, including a popular illustrated series called People’s Books on Natural Science, “a work which I read with breathless attention,” said Einstein. The 21 little volumes were written by Aaron Bernstein, who stressed the interrelations between biology and physics, and reported in great detail the experiments being done at the time, especially in Germany.
In the opening section of the first volume, Bernstein dealt with the speed of light, a topic that obviously fascinated him. Indeed, he returned to it repeatedly in the subsequent volumes, including eleven essays on the topic in volume 8. Judging from the thought experiments Einstein later used in constructing his theory of relativity, Bernstein's books appear to have been influential.
For example, Bernstein asked readers to imagine being on a speeding train. If a bullet is shot through the window, it would seem to have been shot at an angle because the train would have moved between the time the bullet entered one window and exited the other window. ... (and Bernstein continues with additional subject matter familiar to the Einsteinian way of thinking)
Talmud also provided non-popular reading material to the young Einstein :
Talmud also helped Einstein explore the wonders of mathematics by giving him a textbook on geometry two years before he was scheduled to learn that subject in school. When Talmud arrived each Thursday, Einstein delighted in showing him the problems he had solved that week. Initially, Talmud was able to help him, but he was soon surpassed by his pupil. “After a short time, a few months, he had worked through the whole book,” Talmud recalled. “Soon the flight of his mathematical genius was so high that I could no longer follow.”
but this is outside the scope of your question.
EDIT
Note that Isaacson, or the translator, misnames the series as "People's Books on Natural Science". The correct name is "Popular Books on Natural Science. For practical use in every household, for readers of all classes". Here is a link to a PDF. Click on the text to turn the page.
