Good question. Much of his writing comes from his speaking, but he did write several books. However, the only two of size were TSM and The Church and the Work Volume 2.

It's nice to read Church and the Work volume 1, 2, and 3 together in order. This is how I find biblocality undeniable.

Another book to read relating to church assembly is Assembling Together and Spiritual Authority.

I am going to reread through all of Nee's writings again. Only read from CFP and CLC, not LSM.

Just an added warning. You might be compelled at some point to read the Witness Lee's Living Stream Ministry version of Nee's writings which are 30% longer (unnecessarily verbose and inaccurate). Don't do it. Once you have finished all Nee's writings from CFP & CLC if you feel like it you can read LSM only to critique their horrific translations. One example I did here,

http://www3.telus.net/trbrooks/Spiritual_Man.htm

This critique I did is a very fine piece of work. You can really see how horrible Witness Lee's translator was. He was very confused. It will alter spiritual life if you go with the LSM version.