If it's a book written for a national audience, she will be misled. Many plants that are glorious in window boxes in Orgeon and Michigan cook in the Texas heat, even though her window boxes are in the shade.
I can tell you already that coleus and caladiums are going to be her best bets, but there are several books on Texas gardening, gardening in the South, gardening along the Gulf Coast, etc. Is she hear Houston? River Oaks Garden Club has a marvellous guide. There's Dale Groom, Scott Ogden, Neil Sperry- and with trepidation I mention Howard Garrett. His ideas on home remedies, magic sands, and the like are cuckoo, but he might have suggestions for plants that take the Texas heat in the shade.
Oh, some smaller hosta varieties might work, too, if she will be diligent about slugs and snails. Hostas are slug magnets.

She is in Western Massachusetts. I know coleus does well as an annual around here; do caladiums need to be dug in the fall?

absolutely. they are very tender, even in Texas. We don't plant the "bulbs" until mid-May. They can be planted as late as July 1. They will die if left in the ground (or planter box) over the winter. However, the tubers can be saved if dug, and replanted the next year. Any search engine would give specific details.

The neat thing about annuals and perennials is that most of them are adaptable to a wide variety of climates, as long as you have an understanding about their general requirements. I suggest you do a book search on 'container gardening for the shade', 'shade loving annuals and perennials', etc. I wouldn't be surprised if you have a great selection of gardening books in your local bookstore.

You needn't search for a speciality book, but I expect that you will find plenty of titles geared for the northeastern gardeners.

A quick search in that big on-line bookseller's site comes up with several books for container gardens...one mentions windowboxes in particular.

1. Ferns of all types and sizes for shade areas:
http://gardenhobbies.com/flower/ferns.html

2. Hostas of all types and sizes for shade areas:
http://gardenhobbies.com/flower/hosta.html

3. Bulbs
Flowers are more problematic. There are many varieties that thrive in partial shade. Check the back of the seed packet for light needs of the plant.
Bulbs provide a tremendous opportunity to provide color in a shaded environment. Spring flowering bulbs will grow in the spring before the leaves develop. Look for early blooming varieties.

4. Bushes and Shrubs are no stranger to shade gardens. Try Rhododendrons and Azaleas. They are often used as foundation plants around the northern sides of houses.

5. Hydrangeas
are rather easy plants to grow. They prefer loamy, well drained, acidic soil (pH 6.5 to 4.5) enriched liberally with organic matter. Most hydrangeas prefer quite a bit of shade, although H. macrophylla and H. serrata will tolerate much more sun, but will still flourish in mottled sun for part of the day. It is important to keep the roots moist and well fed by covering with mulch; this is especially true with those planted near trees. They may also need protection from spring frosts, which can kill the buds. In some species, the flowers change color according to the amount of aluminum and level of acidity in the soil. H. macrophylla will produce blue flowers in acidic soil where more aluminum is present. The lower the pH, the bluer the flowers. In neutral soil, they take up less aluminum and the flowers are pink. On white flowers, only the eye color of the male flowers will change. To produce blue flowers even if your soil is neutral or alkaline, add aluminum sulfate or sulfur to increase acidity. Plants grown in soil with a pH level higher than 7 (alkaline) can also become chlorotic because of a lack of iron and must be treated accordingly. Pruning should be done in late winter or early spring. This consists of cutting off dead flower heads back to the first leaf node that has buds and dead, weak, or crowded stems back to the base.

PRUNING:
Question: I have just cut my hydrangeas back - some of the old stalks had new buds, and I cut them all back. There are other buds coming from the base of the plant which seem to be fine. I'm hoping I didn't ruin this year's growth - I live in New England - on Cape Cod. What can I expect from the plant this year???

Answer: Unfortunately you may have pruned away most all of next season blooms. The ideal time for most hydrangeas to be pruned is late summer before buds are set. Buds form on 2nd year stems. Information "How to Prune the Hydrangea Types" written by Richard Weir or Long Island Gardening thru contact with Cornell Univ. Co-operative Extension. Use Monty's Joy Juice orange label (2-15-15) on the roots or spray the leaves and see the difference. Two tablespoons Epsom Salts around each plant every 6 weeks during the growing season should prove helpful.