The Book of Love

Background
This quest begins in the Temple of Mara in the city of Riften. It is given to the Dragonborn by Dinya Balu, one of the priestesses, when asking for the Blessing of Mara. To earn Mara's favor, Dinya sends the Dragonborn to the village of Ivarstead to help a local girl with a love problem.

Riften
This quest is easily located by entering the town of Riften and following the path on the left towards Mistveil Keep. The Temple of Mara is clearly marked by a pair of guards standing before an opening in the stone wall with twin banners hanging behind them. Enter the Temple and speak with Dinya Balu, the Head Priestess of the Temple, and request Mara's Blessing. She will say that one must prove themselves worthy of the blessing by advancing the cause of love throughout Skyrim, and relays a vision sent to her by the Goddess to visit a small village called Ivarstead to the west of Riften. Head west to Ivarstead.

Ivarstead
Upon arriving at the hamlet of Ivarstead, they are prompted to speak with a girl named Fastred, who is in something of a conundrum with her parents. Fastred fancies herself in love with a local man named Bassianus Axius, but her parents disapprove of their choice to leave Ivarstead for Riften. After speaking with Fastred, speak with her mother and father as well.

Speaking with the mother will reveal that she does not disapprove of Bassianus, and would allow the pair to elope while she took care of informing her husband. Speaking with Fastred's father will show he thinks his daughter is a rather fickle young woman whose affections change frequently. He will state that, up until very recently, Fastred was infatuated with another local man by the name of Klimmek, and that he is concerned that Fastred will leave the rapidly shrinking village for good if she chooses to leave with Bassianus.

There are two choices: Head to the local inn to inform Bassianus that Fastred's mother is willing to run interference so that he and his love interest can elope to Riften, or investigate Klimmek's past relationship with Fastred. Confronting Klimmek will result in the fisherman realizing that he must be assertive with his feelings for Fastred, and let her know that he wants to be with her. Additionally if this path is taken, Bassianus moves to Riften alone. Speaking with Bassianus will result in Fastred and Bassianus eloping to Riften, never to return to Ivarstead. Choosing either will result in the quest advancing.

Next, return to Riften to learn that the Goddess requires a journey far to the west, to the city of Markarth to stoke the flames of love in the heart of Calcelmo, the eccentric Dwemer researcher and court magician in Understone Keep.

Markarth
Calcelmo is experiencing a difficult situation. He's attracted to a woman named Faleen, who works in the keep, but has no idea how to speak with her. Fearing the possibility of offending her in some way by bringing up the wrong subject, Calcelmo asks the Dragonborn to get advice from a friend of Faleen's named Yngvar, who is reputed to be quite successful with wooing the local ladies.

Yngvar can be found in the Treasury House at night or in the Silver-Blood Inn by day. He is also often found resting against a rock by the stream, just opposite the Abandoned House. If Stormcloaks have taken the city, he will be the new Jarl's housecarl. If Markarth was negotiated for the Stormcloaks during Season Unending and retaken by the Legion, he will be at the Palace of the Kings. He will reveal that Faleen puts on a tough front, but has a weakness for poetry. It just so happens that Yngvar has a poem that he can tweak to be about Faleen... for a sum of 200 gold. Pay the piper and take the poem back to Understone Keep and speak with the lucky lady.

After prompting Faleen to read "Calcelmo's" poem, she will ask that a letter be delivered to him. Head back to Calcelmo, and proceed to follow him and witness his stumbling confession. If Faleen is in Solitude, it will take several hours for Calcelmo to walk all the way from Markarth to Solitude so proceed with other quests until a notification shows up in the quest journal that Calcelmo has spoken with Faleen, or wait for four hours and he will be there. With the job in Markarth complete, return to the Temple of Mara for the next objective.
 * If Stormcloaks have taken Markarth, Faleen will be found in the lower floor of the Blue Palace in Solitude, rather than Understone Keep. Yngvar will be in the Understone keep as the Jarl's housecarl.

For the final task, Mara requests that two lovers be reunited who have lost one another beyond the pale of death.

Gjukar's Monument
Put on the amulet given and travel south-southeast of Rorikstead to find the spirit of Ruki, looking for the body of her husband Fenrig among the ruins. Speak with her and she will ask for help looking for her husband's body. With no corpses present in the area, head northeast towards an open plain. The spirit of Ruki's husband is just east of Greenspring Hollow on the south side of that river.

Speak with him and he will ask to be led back to Ruki. At this point, either choose to fast travel back to Gjukar's Monument, or take the scenic route. After reaching Ruki, the two dead lovers will speak briefly, and ascend towards the heavens as they pledge eternal love for one another. They do not seem to know that they are dead.

Return to Riften and speak with Dinya Balu to receive the Agent of Mara blessing (Permanent +15% Resist Magic) and complete the quest.

Trivia

 * It is highly recommended to complete this quest before the civil war questline or Season Unending, as Faleen and/or Yngvar will not be in Markarth after that. Also, the resist magic is useful against dragons.
 * Even after moving to Riften, Bassianus (and Fastred if they elope) will still have their dialogue as if they lived in Ivarstead. They will still be able to be asked about High Hrothgar.
 * When first encountering Ruki, be sure to exhaust her conversation options in order to continue. Only following the branch about not seeing any bodies may not advance the quest even though she asks you to search with her: the marker will still point to Ruki and Fenrig will not speak to the player.
 * In popular culture, "The Book of Love" is the title of a song originally by The Monotones, and a different song by the same name, by the Magnetic Fields.