The Step-By-Step Encyclopedia of Practical Gardening: Guide to Gardening Techniques, by Alan Titchmarsh. (Published in cooperation with the Royal Horticultural Society.) New York: Simon & Schuster. 96 pp. $8.95.

Up north there's still time to satisfy some of that spring digging urge with this last of a four-part set now in print in the United States.

This excellant book, which the reviewer wishes she had years ago, covers the basics of soil preparation, improvement, and cultivation along with the design of rock gardens and flower and vegetable beds. But it also includes some unexpected topics like making window boxes. Each segment is well illustrated, so even the novice can't fail.

The sturdy spiral style notebook comes in a heavy plastic slipcase which the reader can take right to the garden.

Other titles in the set: ''Guide to Lawn, Ground Cover and Weed Control,'' ''Guide to Growing Under Glass,'' and ''Guide to Garden Pests and Diseases.'' Each is 96 pages and priced at $8.95.