Infinitive or gerund

#3 is correct.

Thanks a lot AlpheccaStars,

What if one turns the sentences around:

1-What I did was read books .
2-What I did was to read books.
3- What I did.

#1 is correct. Now you have a dependent clause as the subject of the main verb "read".
In the first posting, the verb in the main claue was "was", and the dependent clause (what I did) was a predicate object.
The words before "was" had to make sense as a subject. The gerund phrase "reading books" fit as a subject.

AlpheccaStars #3 is correct.

Hi Alphecca! So, in the second sentence "To read books was what I did" cannot infinitive "to read" act as a subject?
I wish you Happy New Year!

navitasan1-Read books was what I did.
2-To read books was what I did.
3-Reading books was what I did.

These are inverted pseudo-clefts. Only #3 is idiomatic. Nevertheless, #1 is sometimes heard.

In the normal pseudo-cleft order (the what

Thanks a lot.

Happy New Year!