The Mabinogion.

[Beorn]

The Mabinogion was a collection of eleven (twelve) tales from the Welsh myths. The tales of the Mabinogion were preserved in two manuscripts, White Book of Rhydderch (c. 1325) and the Red Book of Hergest (c. 1400). Though the Rydderch manuscript was the earlier of the two, the tales of Lludd, Culhwch and Owein survived only in fragments, while the Dream of Rhonabwy was completely lost. Only the Hergest manuscript contained all eleven tales.
The Mabinogion was first translated into English by Lady Charlotte Guest. It was Lady Charlotte who gave the title of "Mabinogion" to this collection of tales. Also, Lady Charlotte had included a twelfth tale, called Hanes Taliesin ("Tale of Taliesin"), belonging to the Independent group. However, the Hanes Taliesin was not found in the two early manuscripts, so some of the later translations of the Mabinogion do not include the story of Taliesin.
The tales from the Mabinogion can be divided into three categories. The first four tales belonged to the Four Branches of the Mabinogi ("Pedair Cainc y Mabinogi"). The next four (or five, if including Taliesin) were the Independent tales, two tales of which Arthur appeared in the scene. While the last three tales falls into a category known as the Welsh romances, similar to those of the French romances written by Chretien de Troyes.
What were the four Branches of the Mabinogi? These four tales were told in the correct order, with Pryderi appearing in all four tales, but who only played minor role in each of the tale. It began in Pwyll Lord of Dyved, with his birth, and then it ended with Peredur's death in the fourth tale, Math Son of Mathonwy.
With the independent tales, The Dream of Maxen involved an emperor marrying a maiden he saw in his dream, while in Lludd and Llevelys, the story involved with Britain suffering three strange plagues. Two other tales involved Arthur and his companions (not from the Round Table). The most important of these two, was written in 1100, called Culhwch and Olwen. The other was called the Dream of Rhonabwy.
The Welsh romances were similar to the popular French Arthurian romances written by Chrétien de Troyes (fl. 1165-1190). Since these three romances belonged to the mainstream Arthurian literature, I have only briefly summarised Peredur, and used Chrétien's tales instead of Owein and Geraint from the Mabinogion.

Source

[Treffie]

Thanks B, you've got there before me!

It's a very difficult book to read, but I've managed to read it from cover to cover. (In modern print of course)

The White Book of Rhydderch



The Red Book of Hergest.

[Beorn]

It's a very difficult book to read, but I've managed to read it from cover to cover.

When you say that, I presume you mean in actual Welsh as compared to English?
I find it hard enough to pronounce the names. I usually give each character a shortened nickname to ease the reading.

[Treffie]

I've read it in both English and Welsh, but personally I prefer it in Welsh as nothing gets lost in translation. The English version is good but there are differences - not in the detail, but the general feeling.