The Totally Un-Authorized Book of Tuning Yo-Yos by Sky Kiyabu, published in 1999 by the Downtown General Store in Honolulu, Hawaii.
Along with the yo-yo boom of the late 1990s arose the boutique craft of modifying stock yo-yos. There were Web forums that discussed yo-yo modding back then, but to my knowledge this booklet was the only printed and published guide on the topic. It covered modding techniques for some of the most popular yo-yos at the time. It was all in an effort to customize the playability of a yo-yo to your own liking, and to increase spin times so you could perform increasingly sophisticated string tricks. Along with yo-yo modding came the need for a wide variety of yo-yo parts. If you look at the printed retail product catalogs from popular websites around this time period--such as Infinite Illusions (aka yoyoguy.com)--many of them included extensive parts lists.