The following are books (along with publisher and ISBN) I have found which have Celtic designs in them:
CELTIC CHARTED DESIGNS - Co Spinhoven - Dover 0-486-25411-9
CELTIC STENCIL DESIGNS - Co Spinhoven - Dover 0-486-26427-0
CELTIC ANIMAL CHARTED DESIGNS - Ina Kliffen - Dover 0 - 486-29125-1
CELTIC CROSS-STITCH - Mike Vickery - Sterling 0-8069-1382-7
BOOK OF KELLS - ed. Blanche Cirker - Dover 0-486-24345-1
The Vickery is hardback and printed in color on quality paper. Designs are not literally taken from Celtic sources, but, I think, are his takes on these designs. They've been "modernized" somewhat and don't have that archaic "edge" to them that the originals have. Complete illuminated alphabet. Colors are quite vibrant. Consultation of my facsimile copy of the BOOK OF KELLS shows semi-vibrant colors, but no doubt these have faded with time. Surprising use of primary colors, though: red, green, blue, yellow, brown, purple. Lots of gilt, too, of course, and black. Vickery uses blue-greens, oranges, beiges, etc. that I don't find in KELLS.
The Kliffen book is primarily animals. Some scroll- and knotwork. Hand-drawn graphs. B&W charts, but floss color suggestions are given. Again, these don't square too well with my KELLS. It may be that the artist's suggestions are not really based on artifacts.
Spinhoven stencil book has many excellent knots, scrolls, braids, borders, medallions, as well as "humans" and "animals." Place a piece of graph paper on top of B&W images to produce designs (or run it through a scanner and your favorite cross stitch charting software).
Spinhoven charted book is preponderantly geometrics, as in the stencil book. Some pictorial pieces, and these would make up quite large unless a high-count fabric were chosen. A couple pages of "beasts". B&W hand-drawn charts. Unfortunately, no color suggestions. Suggest you consult a facsimile of KELLS. Also consider Dover's $1 book of bound postcards based on KELLS.
The KELLS listed above is not the facsimile edition I have, but the price is right, and if you're interested in things Celtic, it's worth adding to your library. Dover also has some books of Celtic motifs in line art format: 159 CELTIC DESIGNS (Lusebrink) and CELTIC DESIGNS AND MOTIFS (Davis). These are on my to-purchase list and look like they'd be very useful for converting to charts.
I have charted two Celtic (uncial) alphabets in backstitch and will be putting these elsewhere here on my home page when time permits. Still looking for Celtic alphabets. These are few and far between, and believe me, I've looked! There is one Celtic-ish alphabet appearing in Sep. 95 issue of For the Love of Cross Stitch; it is on the cover.
copyright 1996, Martha Beth Lewis
Contact me about reprint permission.