The Musicarta Pentatonics Workbook
The pentatonic scales have just five notes - two of the major and minor scale-tones are missed out. They're easy to learn and generate lots of music, from heavy metal through mainstream rock jamming to folk tunes. You'll recognise them the instant you hear them - just browse the MisterMusicarta YouTube Pentatonics playlist - https://www.youtube.com/playlist?list=PL5JLvea_G1xfrmjvYY9hgHTRxBXXZBasl - for proof.
Pentatonic scales are great for improvising, because they provide a limited set of notes which sound good over a maximum number of chords. Only five notes to choose from, and virtually none of them 'wrong'!
This makes them ideal for practicing finger dexterity and speed - it's as close to 'practising improvising' as you can get, and the Musicarta Pentatonic Workbook has enough examples to keep you going for years.
The simplified and familiar pentatonic scales offer an excellent opportunity to develop your 'playing by ear' skills, too - there's a whole coaching module in the workbook dedicated to the task.
Browse the sample content pages - http://www.musicarta.com/pentatonic-scales_1.html - at leisure before purchasing. You can even take the first module for free.
If you're interested in boosting your popular-styles keyboard skills and savvy, the Musicarta Pentatonics Workbook is an ideal place to start.