Acquisition of Spells Beyond the 1st Level (DMG)

Once a character has begun adventuring, he won't be able to have additional spell books instantly appear each time he goes up in level. Instead, the player character must find some way to get additional higher level spells. As with initial spells, there are several ways this can be done. Any or all of these can be used in your campaign.
===Gaining Levels===
First, whenever a character attains a new spell level, allow the player one new spell immediately. You can choose this spell, let the player choose it, or select it randomly.
The rationale behind this is simple: All the long hours of study and reading the character has been doing finally jells into something real and understandable.
No roll is needed to learn this spell, unless you allow the character to choose it. If the character is a specialist in a school of magic, the new spell should be from that school—if there is a spell available.
===Copying from Spell Books===
The second way to acquire new spells is to copy them from the spell books of other wizards. A character can copy from other player characters (if they will allow it), pay NPC wizards for the privilege (see Chapter 12 : NPCs), or take them from captured spell books. When copying spells, a character must roll to see if the character can learn the spell. No character can copy without magical aid of a spell of a level he cannot cast.
===Scroll Research===
Third, a character can research a spell using a scroll with the same spell as a base. The time and cost required for the research is half normal and the player character must still check to see if he can learn the spell. Regardless of the success or failure of the research, the scroll is destroyed—the wizard had to read it aloud to analyze its effects.
Scroll research cannot be done in an adventuring situation. The wizard must have carefully controlled conditions even to attempt it.
===Study with a Mentor===
Fourth, and only if you allow it, the wizard can return to his old mentor and, with luck, copy a few spells out of his master's spell book. Use this method if, and only if, you feel it is important for player characters to have more than a few new spells each time they advance to a new spell level. Allow the characters to gain too much this way, or too frequently, and they will come to rely upon it, not using their own playing ability to develop their characters.
==DM Control of Spell Acquisition==
However characters acquire new spells, always remember that you are in charge. You have complete control over what spells the player characters get.
If a player character has a spell you don't like or one that severely disrupts or unbalances your game, it is not the player's fault. Who gave the character the spell? Who allowed it in the game? Controlling spell acquisition is an important responsibility. Consider your choices carefully.
By keeping the selection of spells limited, you automatically increase their importance and value to the wizards in your campaign. A simple scroll with a single spell becomes a real treasure if it has a spell on it the wizard has never seen. This gives the player a touch choice. Should he cast the scroll during an adventure where it might be useful? Should he save it until he can take the time to research the spell for his spell books?
When the characters overcome a hostile mage, the first concern of the wizard will be for his spell book. Where is it? What spells does it have in it? Even a nonmagical item like a spell book becomes very important. Knowing their value, NPC wizards will go to great pains to protect their own spell books, hiding them carefully, locking them in trapped chests, and scattering magical traps throughout the pages.