The dog days of August could be an ideal time to get started on a project that will keep you out of the sun, captivate the children and turn into a treasured gift or an ongoing family activity.

While others complain of end-of-summer ennui, why not start assembling a book--plain or fancy--that celebrates your family?

Perhaps one of the most ambitious versions around is the elegant tome prepared by Nancy Merritt, program director of the Alexandria Community Y. She spent four years and $4,000 putting together, Heritage: Six Generations of the Merritt Family, which she presented to her family last Thanksgiving. The 109-page hardcover book contains photographs, drawings, maps, documents and text that tell the history of the Merritt family from 1800 to the present.

Merritt, 31 and single, wrote, illustrated and published the book both for the enjoyment of her family and to get in touch with her own roots. Her parents, says Merritt, "were so touched. We spent the next three days talking . . . about World War II . . . about how the family pulled together . . . story after story. Knowing how they struggled makes it easier as adults to meet challenges.

"Unless people have somewhat grasped their place in a family network," claims Merritt, "it could be difficult for them to feel spiritually grounded. It is not important how great a family one comes from, or how successful, by the world's terms, but that every person is a continuing spiritual force in a long, ongoing process. The key is in recognizing the importance of every person's story."

Although Merritt's book is elaborate and professional, yours--regardless of how simply done--will be equally valued. Consider, for starters, what you have stuffed in drawers, shoe boxes and attics: photos, letters, children's drawings and schoolwork, deeds, certificates, newspaper clippings, ticket stubs, programs, post cards and much more. By assembling these gems in a loose-leaf album, you can make a unique "family book."

Young children are intrigued by family relationships and the idea that their parents and grandparents were once young. They ask things like, "Grandma, who were your children?" and "Dad, do you mean Aunt Susie is your sister?" With family members scattered all over the globe and many grandparents living in a condo, rather than the old homestead, it is often difficult for children, or even adults, to see where everyone fits in the family.

So that it does not become overwhelming, you might consider the book as a continuing project and build your collection one part at a time.

Regardless of whether the book is for a child, brother or sister, a parent, or a grandparent, try to show the strength and durability of the family--not merely its accomplishments in material ways. Commendations are fine, but be sure to include some sentimental and silly things that show these are normal people who worked, loved, struggled and had fun along the way.

Buy an album with peel-back plastic pages that protect the contents. Many department stores have large, 30- to 50-page albums on sale for about $12 to $15. A loose-leaf style is best because it can be expanded or rearranged easily. Beware of very cheap albums, for the sticky page coating can dry up, and in a year or so, the contents will fall out.

Have the eldest member of the family hand-draw and write a family tree. Spaces should be left for additions. The book should have a continuity--great grandparents to grandparents to parents to children to grandchildren--showing that each person is many things in life. A young child will not grasp this concept immediately, but over the years, the book will help him or her see the family as a complete and continuing unit.

A map that pinpoints where family members live, have lived, or originated can be interesting. Maps can be photocopied. Include old family pictures as far back as you can find. Every person need not be shown, but each generation should be represented. The most expensive and time-consuming task is the reproduction of old photos that lack negatives. Decide which family photos are essential; borrow prints from other family members, if necessary. A new negative for a black and white or color print ranges from $4 to $15, plus the cost of the new print. Sometimes black and white is more expensive than color. Compare prices.

"Direct copy prints" are available from some area photographic stores. At Snap Shops, a 35mm color print can be copied without a negative for 89 cents. Additional copies of the same print are 69 cents. An 8 x 10 color print can be direct-copied for $4.29. One week to 10 days is required for these services.

Because your photos are irreplaceable, deal with a reputable lab to help insure against loss of your print.

Photocopying is a boon to makers of family books. For about 10 cents a page, high-quality, instant, black and white prints can be made of family treasures. If an original is beat up, a photocopy will make it look as good as new. Items can be enlarged or reduced. This is helpful for young artists' first drawings, which tend to be large.

Material from yearbooks and other books can be photocopied. Back issues of newspapers can be photocopied at libraries. About five standard photos on one page will cost only about 10 cents. Photocopies are not as clear as reprints, but, they are acceptable and increase the material you can include at a reasonable cost. Use a printer with a top-quality photocopying machine.

Draw on the talents of family members. An artist can draw a sketch of the family home, grandma's kitchen or the layout of the home town. A person who prints well can prepare titles. You may choose to write a short text that describes people or events in your family, or you an limit the book to pictures and mementos.

However you put your book together, you can be sure it will be treasured for years to come. But be warned: When the rest of the family sees it, they'll want one. Be prepared to offer material for duplicating.