Lester Leaps in is a “rhythm changes” tune written by Lester Young (tenor saxophonist in Count Basie’s band) in 1929. Below is a video of Lester Young playing this tune with Count Basie’s Kansas City 7 in 1939. In the Depression, and subsequent World War II, big bands wound up playing with their core personnel and swing started to move to the Bebop style, as evidenced by Count Basie’s big band becoming the “Kansas City 7”. Benny Moton, Basie’s bass player, also pioneered the concept of a walking bass line and locking in with the rhythm section. At this time, they were still playing major six chords in lieu of major 7th chords, as evidenced by this recording.
The song is in AABA format, with 8 bars each section. There are two ways to look at the bridge – one that goes to the III7 Chord, followed by a cycle of 4ths. Another way to look at it is cycling through the secondary dominants, starting with the V7/vi chord. Some rhythm changes tunes go to the ii V7 of the VI instead of the V7/vi. There are many songs which hold these changes, the first one we’ll learn is Lester Leaps In.
This website his several resources for you on how to play this tune.
First and foremost, I encourage you to learn the melody from the recording. You can use a real book, but sometimes the chords are wrong, so I advise to use the recording first.
Once you’ve learned the melody and play along, I would advise you to start learning the chords.
After learning the chords, I would advise to learn the two note voicings for piano proficiency and for use as guidetones. I would also recommend practicing a bass line, which you can play while others are soloing if you are in an educational setting.
Once you have internalized the chords and you can play the bass line, practice the two note voicings on the piano with the recording. Listen to the pianists comping for ideas about rhythm.
Hope this is helpful!