The word etude means “study.” The purpose of a piano etude is to help a pianist develop a particular skill such as playing scale passages or arpeggios fluently, learning proper chord fingering, getting familiar with a particular key, playing new rhythm patterns, executing a certain kind of articulation/phrasing technique, etc. As such, they tend to have a lot of repetition, are generally on the shorter side, and are oftentimes marked allegro (fast). There are many collections of etudes, the most famous of which are Chopin’s (brilliantly difficult!) Etudes. Many young piano students are introduced to the works of Hanon, Burgmuller, Heller, Czerny, Maykapar, Cramer – and more. Advanced difficulty level etudes include those of Scriabin, Liszt, Rachmaninoff, and Chopin. There are many others we could list. I suggest you browse IMSLP.org if you’re interested in viewing some of these traditional scores.
With adult students, I rarely assign traditional etudes. Instead, we work with multiple sequenced and graded collections, particularly those of William Gillock. Other recommended books include Multikey Reading by Jane Smisor Bastien and Victor Labenske's Piano Miniatures, along with the Faber Adult Piano Adventures books. Sometimes we also supplement with other adult methods, including Alfred's All-in-One or the Hal Leonard Adult Method. To become a proficient reader, it's crucial to have a large selection of multi-leveled reading -- things that can be read "at sight," things that can be learned in one or two weeks, along with one or two more difficult pieces which could be performed after approximately 8-12 weeks of practice.
In order to build the skills necessary to play the more difficult pieces, we almost always create our own exercises using the music itself. Whenever we employ techniques such as staccato articulation, rhythmic variations, “blocking” broken chords, playing “at the points of coordination,” making “flashcards,” etc., we are creating our own miniature etudes, the purpose being to master the technical skills necessary to play the chosen piece. I prefer using this technique with adult students as I feel it is the most efficient use of your practice time. You may already know that most of the collections we work with, most notably all-things-Gillock, are designed for the purpose of building general pianistic skills: familiarity with all major and minor keys – both in reading and “feel”, a solid understanding of rhythm/timing, good fingering technique, execution of various pianistic articulations, understanding how to properly play in various “styles” (according to the music period). Separately, we work to have knowledge of music theory and an understanding of music terminology (frequently indicated in Italian).
There are many etudes that may be appealing to adult learners. Here are just a few suggestions for early-level students: William Gillock’s Sailing Home (D Major) or Evening Sunset (A Major), Chopin Etude Op. 10, No. 3 (Arr. W. Palmer) from Alfred’s All-in-One Course - Bk 2, Carl Reinecke’s Elegy from Keys to Stylistic Mastery, Bk 1, Stephen Heller’s Tolling Bell from Jennifer Linn’s Journey Through the Classics (complete). Intermediate level students might enjoy William Gillock’s Faded Letter (E Maj) from his Preludes in Romantic Style or the dramatic Deep Blue Sea (B Min) from his Accent on Majors & Minors. Short ‘n Sweet (B Maj) from Victor Labenske’s Piano Miniatures is another very manageable and enjoyable short selection. If you have a duet partner, you might want to check out Naoko Ikeda’s Duets in Color, Book 1 (Majors), and Book 2 (Minors). These are intermediate-level duets that do live up to their “colorful” title. Student favorites include Midnight Blue Ballade (f# min) and Golden Beaches (Db Maj).