I've read all the Ultima books, both the clue/hint-type (which sometimes fictionalizes the info) as well as the pure lore fiction books.
The ancient Ultima Saga books were interesting and entertaining, but were somewhat technically lacking in my opinion. Anyone who hasn't played the canon I-V might be a bit lost. Good luck even finding these books...
The Technocrat War I have very mixed feelings about. They were well written and full of UO2's lore (some of which was shoehorned into Ilshenar), but parts of them are as dry and uninspiring as a desert (descriptions that ramble on, and on, and on). While finding these books online isn't too difficult, be prepared to pay through the nose for a decent copy.
Sadly, all the Ultima fiction books are out of print and no ebook versions exist.