I've owned 501 Spanish Verbs paper edition for years and found it very helpful. When it looked like it was formated for Kindle I bought that one too. It's good but it has some flaws. The complete introductory grammar discussion is easy to read. The "501" verbs are easy to access and easy to read.
But the other two indices have been a problem. In the paper book you can use the English Spanish verb index to look up English verbs, find the equivalent in Spanish and see where the conjugation is. Also you can use the 2,200 verb index to look up Spanish verbs not included in the 501 for a short definition and a similar 501 dictionary entry to help you conjugate it.
In the Kindle edition it's easy to use the Table of Contents to get to the first page of these indices. But to actually use them you would have to page through, page by page, until you found the word you want. There's no way to search for anything except the 501 verbs.
I got around this problem by creating a "Note" at the beginning of each letter in the two indices. When I want to look up an English verb, I "View Notes & Marks" and can page through quickly to the right area.
What the book really needs is two more searchable "dictionaries". I hope that if it's ever developed the upgrade will be available to us, the pioneer users.
Note: I've made my Notes public so you may be able to use them to access these indices with having to enter them yourself.