A yellow Slayer tome is a experience reward from the minigame, Temple Trekking. You can get one by taking one of the medium followers to Paterdomus or to Burgh de Rott. At the end, you will get a Reward token (yellow) which you can exchange by clicking on the first option, Claim-reward. The Slayer tome grants Slayer experience once read, and the blue version will give 2035-3025 Slayer experience.