The “Be Series” commentaries by Warren Wiersbe are a favorite of many, including me. The series consists of 50 volumes with titles like Be Confident (Hebrews), Be Right (Romans), Be Resolute (Daniel), etc. The individual volumes are usually available in paperback or Kindle ebook format on Amazon for about $10 each. At the moment, at least
eight five volumes of the Kindle edition are on sale for 79¢ to 99¢ each through either the Kindle app, Kindle readers or the Amazon website. Those are Hebrews, Ezekiel, Deuteronomy, Genesis (12-25), and Acts (13-28). Purchasing them on sale now would cost under $5.00 versus about $50.00 regular price. You can click here if you want to check the website to see what is available. At the website ask to sort results with “Price: Low to High” so the low priced items will appear near the top. Also look only for those titles beginning with “Be.” (Updated 08/14/2019)
Over about two years I was able to purchase the entire 50 volume series in Kindle format just this way. It just required checking at least weekly in the Kindle Store. Remember that you can then read all purchased books on any tablet, phone or computer which has the Kindle app available.