Merch of any kind for classic anime Serial Experiments Lain is very hard to come by. Apart from "Omnipresence in Wired" (another essential book for Lain fans, with loads of beautiful illustrations by legendary artist Yoshitoshi Abe), there is this book, somewhere between a guidebook and encyclopedia for the anime in particular. It's laid out by episode with character profiles and similar as well as thoughts of the creator and production notes. Even if you can't read Japanese, it's worth picking up just to have, since Lain merch is so hard to find (and most is exceedingly rare and out of print or out of run.)