Book Recommendations on Private Credit?

Unfortunately there doesn't appear to be an abundance of great literature of vanilla private credit, but I can point you towards a couple resources, most of which are actually geared toward leveraged loans. 

The first would be the S&P LCD Leveraged Loan Primer. A must read that provides a great base of knowledge about leveraged loans, which can more or less be considered one liquidity-rung above private credit. Additionally, if you have access to LCD through your job, reading some of the articles can provide good, practical background on what's actually going on in the market (particularly the monthly/quarterly reports they publish).

There's a book called Private Debt: Opportunities in Corporate Direct Lending by Stephen Nesbitt that is worth a quick read. Nesbitt is not an actual direct investor, rather a consultant, so this book is written more from the point of view of PC as an asset class rather than providing instruction on actually doing the lending. Still, it's helpful perspective on topics that I wouldn't have really thought about otherwise, such as how yields can be considered PC beta and loss rates are the alpha. 

Leveraged Financial Markets is quality; each chapter is written by a different author and covers a different topic, so it's amenable to hopping around. Good chapters on credit analysis and indentures. I know you didn't ask about distressed, but the chapters on CDS and DIP financing are some of the best at simple distillation of otherwise complex topics (most books on distressed don't really delve into either of those topics particularly deep). 

A Pragmatists Guide to Leveraged Finance is of similar quality, probably better the former. Good chapters on structural issues (structural subordination) and covenants. 

The LSTA's Complete Credit Agreement Guide is probably the go-to for credit doc-related info. It's a slog to read but is good for reference. A pdf copy might be best so you can ctrl+f for any provision you want to read about. 

There might be one or two other non-distressed focused books worth reading but I can't recall at the moment. A lot of law firms have articles about specific covenant/structural/legal considerations that can usually be found via google search.