A Fishing tome is a experience reward from the activity, Temple Trekking. You can get one by taking one of the followers to Paterdomus or to Burgh de Rott. At the end, you will get a Reward token which you can exchange by clicking on the first option, Claim-reward. The Fishing tome grants fishing experience once read. A higher level reward token will give more fishing experience.
The colour of the tome depends upon the level of the reward token.