Your patron is an arcane being of great power, whether its a wizard of great renown, a particularly studied sorcerer or a litch whom doesn't want to bestow upon you power over the dead, however they expect you to put in the work to learn your magic, with some assistance of course.

Gain access to the wizard spell list, the ability to copy and use unknown spells using arcana and a spell book that works like a wizard spell book. In essence its a wizard warlock.

Magical Secrets (Extended Spell list)

At 3rd level you learn 4 cantrips from any casters spell list, they count as warlock cantrips to you, but they don’t count against the  number of cantrips you know.

Additionally, at later levels, you learn the following spells.

Eldritch Study

Starting at 3rd level, you gain access to a spell book, you may use the spell book to copy down spells, these spells must be at a level equal or lower to your spell slots. You may then use these written spells by making an arcana check and expending a spell slot, the DC of which is 10 + Spell level. If you fail the skill check the slot used is wasted. Any spell attacks made with your spell book's spells use your Warlock spell DC and to hit bonus. You may use any spell book or grimoire as an arcane focus.

For each level of the spell, the process takes 4 hours and costs 100 gp. 

You also gain proficiency on arcana checks

Quick Ritual

You may borrow some power from your patron temporarily. Starting at 6th level, you can cast a Ritual spell that you have prepared using an action rather than the extended time for a ritual. Doing so does not expend a spell slot. You may only use this benefit once per short rest.

Eldritch Knowledge

Turns out, being able to harass a powerful arcane entity for knowledge is pretty useful. Also at 6th level, you may use Charisma instead of Intelligence for any intelligence check, you also gain expertise with arcana.

Arcane Connection

Your connection to the weave is deepening. At 10th level you may cast a single spell without expending a spell slot. This feature may only be used one time per short rest

Favourite Spell

You have studied all of this time, now its time to graduate. You may pick any 1st or 2nd level spell from the wizard or warlock spell list, you may cast that spell without expending a spell slot, additionally if that spell was written in your spell book it no longer requires an arcana check to cast.