Activating a saga before using a Logbook will force that encounter to one of the islands.

You can activate multiple Sagas at once to force more bosses. Uhtred is good to force if you want a chance at the Depleted Mana Rune which is needed for Runeseekers Call. Be careful using Boss Sagas when using Aldur's Saga, due to the risk of losing Grand Expeditions.

As of Patch 0.5.4b boss Sagas are supposed to prioritize replacing regular islands over GEs, using multiple Sagas and you may still risk losing GEs, but it should be very unlikely unless you have many GEs and use all 4 boss sagas.

Activating Aldur's Saga before using a Logbook will add a special modifier to each Grand Expedition. These mods can be very powerful, but since the Saga is expensive it should only be used on suitable areas with as many Grand Expeditions as possible. By following the information on this site, and finding an area with at least 4-5 GEs, you are very likely to make a good profit. Still, if you are not into the hunt of good GE area or possible rng-game, just sell the Saga for a sack of divines.

While you can use Visions of Paradise to play a GE twice, it will clear all the special mods on the GE, making it pretty useless

Possible mods can be seen below.