I'm reading the Yoga Sutras. They are short and are meant to be taken in slowly, and meditated on (I suppose). There are also many commentaries available on them. There are also many translations--sadly, I think a fair amount gets lost in translation from the original Sanskrit.

They translate "yoga" as "Union" here, I notice. I think it may be British... they use "fancy" where I've seen "imagination" before.

An idea here seems to be, that until we are able to clear our minds and see what is true, we are not living in reality.

From Sacred-Texts:

1.1 Now, instruction in Union.

1.2. Union is restraining the thought-streams natural to the mind.

1.3. Then the seer dwells in his own nature.

1.4. Otherwise he is of the same form as the thought-streams.

1.5. The thought-streams are five-fold, painful and not painful.

1.6. Right knowledge, wrong knowledge, fancy, sleep and memory.

1.7. Right knowledge is inference, tradition and genuine cognition.

1.8. Wrong knowledge is false, illusory, erroneous beliefs or notions.

1.9. Fancy is following after word- knowledge empty of substance.

1.10. Deep sleep is the modification of the mind which has for its substratum nothingness.

1.11. Memory is not allowing mental impressions to escape.

1.12. These thought-streams are controlled by practice and non-attachment.