I did a completely comprehensive uncomprehensive search myself while autopiloting 20 gates in gallente hisec. I was able to find NPC sell orders for every skill book I looked for (364 days). While the npc university corp (caille, in this case) had all the basic skills, all the advanced skills I looked for (titan, carrier, dread, hac, assault ship, exhumer, science skills, exhumer, logi, jf and others) were all available.

The t2 ship skills has npc orders at the center for advanced studies corp, and fed navy academy. Some, especially science skills also had sell orders are the research agent corp (roden).

While my search was not exhaustive, barring specific skills I missed, I'm pretty certain all skills are available on npc sell orders in hisec. You will have to go to the advanced studies/navy academy for the racial t2 ship/weapon books. Some skill books such as the advanced connections skills, drone spec are sold by the LP stores.

Specifically for industry skills, I was able to find everything. Shady skills like gas cloud harvesting and drug manufacturing are not sold by npc, and are obtained from npc pirate drops. You'll have to buy those from other players.