Since I made this somewhat crude Intelligence guide for someone who asked advice of how to raise Intelligence I will copy paste it.
I am gonna mention all instances of when intelligence goes up important notes before we start. In order to keep track and not double post I will put the map numbers in () Just ignore it. Also things might be out of chronological order. Here we go.
1) (203) Give Brunhilde 5 Heart Branches for +5 Intelligence
2) (184) Automatic +5 Intelligence after fucking with Qum
3) (179) Enter happyville with -80 corruption or less for +5 Intelligence
4) (177) As you are heading for the cabin in order to face with (B)Ingrid for the first time, mess up with the story as much as possible. Don't do what the Dreamer wants during the confrontation with (B)Ingrid +10 Intelligence
5) (123) Don't loot anything for +5 Intelligence
6) (072) Talking to Frieda for + 10 Intelligence
7) (056) Ask the Fairy Godmother at the Red Arena for an intelligence boost once you finish the villager cup +5 Intelligence
8) (046) Before you escape Ulfina's voice for the first and try to talk to the men blocking path 3 times. +5 Intelligence
9) (036) Buy a potion of Intelligence at the Ignored Beggar for +5 Intelligence
10) Also at the Ignored Beggar, speak to the man (almost) opposite of the barman. + 5 Intelligence. (120 Intelligence required)
11) (033) +15 Intelligence from all books of Hoodton Library (100 required for easy, 120 intermediate books, 150 for the hard ones)
12) (007) +10 Intelligence from Bangvile library. (5 from reading them alone, 5 rereading them once reuniting with Hood)
13) (004) This is a bit tricky. Tl;DR Try to interact with everything before meeting Hood for the first time. +5 Intelligence
14) Bimbofication potion + 10 Intelligence
15) Examine the bunny + 5 Intelligence