In the catalog, check the LOCATION: Maybe the book is "available" because it's on Reserve or in Science Reference or at another JHU library. (if the book is not in the library building where you are, you can REQUEST it, through the catalog.)
In the catalog, check the CALL NUMBER: If it says QUARTO at the end of the number, that means that the book is bigger than the others, and it's located at the end of that call number and before the next call number. That is, a book in the RA section that is a QUARTO will be all the way at the end of the RA's and right before the RB's.
In the catalog, if the book's call number says FOLIO, and its location is Eisenhower, this book is even BIGGER than quartos, so it is on the shelf AFTER the quartos, but before the next call number begins.
Also look in the STAGING AREA: MSEL Levels B, C, and D have a big red sign across the main aisle that says "STAGING AREA." This is where the shelvers organize the books that they pick up from around the library. You may go into that aisle and take your book if you find it.
Look at BOOK TRUCKS: If there's a book truck with books on it, look through them and take your book if it's there.
If you've done all of this and still can't find the book, email circmail@jhu.libanswers.com or ask the person at the Service Desk to put a "search" on the book. Over the next few weeks they will look for it and let you know if/when they find it.