I’m not sure why you’d think that customer service would suck. That was the one aspect of retail that I actually enjoyed. (There are not really a lot of mean or rude customers at a bookstore–it is not like some places where the customers are always criticizing the merchandise or where the prices are negotiable, inviting haggling.)
If you really do think you’ll not enjoy customer interaction, ask them if they have a spot in receiving. That job can be tedious, (opening boxes, matching bills of lading or invoices to the expectation lists sent out by the front office (or the purchase orders if it is a small independent), then ensuring the numbers (tracking and price) all match), but you only have to talk to the manager and the UPS guy.
As for floor work, the only truly dreadful part of the job is pulling returns (especially in January when there are billions). The contracts with the publishers treat the hard cover and trade paperback books as if they were on consignment. When a certain period has passed and the book has not sold, the unsold books are pulled off the shelves, boxed up, and sent back to the publisher. (Mass market paperbacks have a similar agreement, except that no one wants to pay the cost of shipping them, so you strip off the front cover to send back as proof that you had not sold the book and destroy the rest of the book.)
Now pulling returns does not sound like a terible deal, but this is how it works. The computer at the front office has tracked how many copies of each book that was shipped to you has gone out through the check-out. Books that have not sold in x days are then reported to the publisher who authorizes a return. The computer then spits out a list of books that need to be picked up and shippped back. This works fine when the title is The Two Towers. You should already be familiar with that book, know the section it is in, and be able to count off the number of eligible returns. However, if the title of the book is actually The Thirteen Best Ways To Meet Your Deadlines in the Office, you can be dead certain that the computer will truncate the title to The Thirteen Best Ways To M, leaving you to guess what the actual title might have been: The Thirteen Best Ways To Meet Your Date, (found in Sex Ed or Personal Health), The Thirteen Best Ways To Meet Your Debt, (found in finance), The Thirteen Best Ways To Make Your Quota, (Business? Sales?), The Thirteen Best Ways To Master Language, (Reference? Education? Self-Improvement?), The Thirteen Best Ways To Murder Your Boss, (Mystery? we would hope, Business?), etc.
And, of course, the book might just have been sold, yesterday while the computer list was printed last week, so an hour of searching turns up nothing. (And after you get yelled at for wasting time because the assistant manager saw it sold, yesterday, the manager will yell at you in three days 'cause he saw it crossed off the returns list while he knows that it is in the store (since the buyer is going to return it, tomorrow).)
But aside from the poverty, I actually almost enjoyed working there. (I am not a retail person.)