To begin, it is dependent on what you want to learn about “Business Level Japanese.” Are you attempting to concentrate on more interactive business Japanese for usage in company settings? Or are you attempting to enhance your knowledge of business Japanese content? If that’s the case, having a solid grammatical understanding so you don’t have to look it up on the internet as often, as well as increasing your ability to read and understand Kanji conjugations, meaning the individual Kanji’s meaning and readings so that you can read & say aloud the kanji conjugation (e.g. in a meeting) and infer the meaning of the Kanji that you say aloud, despite not knowing the exact English translation.
To answer the main question, if you’re looking for business-Japanese content aimed at providing you with a basic fundamental understanding of Japanese used in the workplace, look for role-play Business Japanese textbooks, and if possible, practise the scenarios with a partner, or practise the scenarios yourself.
Look for reading materials utilised by university students looking for work. (就活)? Because those materials are aimed at business novices, I believe they are the greatest resources available online. For example, this site tells the fundamental respectful expression (敬語). And this site contains a terminology list (業界用語) employed in consulting firms
If you go to a bookshop, there are also books for job-seeking students. Learners who are presently working on their business Japanese can also read a range of resources published in Japanese that are available online for 新入社員.
Digital Sense is a good resource that focuses more on business manner, etc, but also has a bunch of Keigo: http://www.digital-sense.co.jp/cc_new/sub/03_8.html
Books are a great way to learn however, to really improve your business Japanese, you do need a tutor. Reading is one thing, but speaking is quite another, and it requires a great deal of role acting and practice.
Also, try Googling for resources for Japanese folks (such as for new workers
新入社員) as opposed to those geared for those studying Japanese as a second language. You may utilise the same tools that Japanese high school and university students do to master formal email writing and business language!
For actual business Japanese use the しごとの日本語 series.
https://www.amazon.co.jp/s?k=しごとの日本語&crid=31EV70J7MZ0G4&sprefix=しごとのにほ&ref=nb_sb_ss_i_0_6
I can only vouch for the email one, but アルク produces excellent materials.
And here’s a free website that’s best for business manners in general.
http://www.digital-sense.co.jp/cc_new/sub/03_1.html
It is geared for 新人 so it should cover basics.
https://www.amazon.co.jp/しごとの日本語-メールの書き方編-奥村-真希/dp/4757413688
There’s an entire series so pick the book which has the most relevant information for you. It should be this one and 電話対応.
