Harlan Ellison taught me a lot things about writing. No, I wasn't a student of his I first read Dangerous Visions was I was 19 and his introductions to the stories really turned me on to how the writing process works and how writers come up with ideas. I read as many and as much of Ellison as I could. I watched interviews with him and even the documentary Dreams with Sharp Teeth. So, I thought I knew a lot about Ellison and his work. That is until I read Harlan Ellison: The Edge of Forever. The authors bring up lesser-known works and periods in Ellison's career such as his work on men's magazines in the early 60's.
The Edge of Forever is a critical work, and it confirmed some ideas I already had about Ellison as well as bringing forth other idea that I either hadn't considered or hadn't even conjectured on.
Even if you think you know Harlan Ellison this is a book for you to read, you will see things you hadn't seen before in his work.