While some product category sales fell for e-commerce giant Amazon.com Inc., others surged this month, according to a new report from One Click Retail, an e-commerce vendor that tracks pricing on Amazon.com 12 times per day across millions of items.

One Click Retail tracked the top five categories by month-over-month dollar sales growth on Amazon for September. The vendor tracked items sold directly from Amazon and those sold by outside merchants via the marketplace. Below are the rankings.

• No. 1: E-books

Incremental sales of e-books on Amazon, No. 1 in the Internet Retailer 2016 Top 500 Guide, totaled $32 million in September compared to August, One Click Retail says, a 73% increase. “The gap between physical and digital books narrowed significantly in September,” the pricing vendor write in a blog post. Sales of paper books dropped by more than half month over month while e-books rose by 73%, largely driven by digital sales of “The Girl on the Train: A Novel” and the still well-performing “Harry Potter and the Cursed Child.” Additionally, One Click Retail reports that the e-book edition of “The Girl on the Train” is experiencing an uptick in sales in advance of the movie opening on Friday.

• No. 2: Shoes

Sales of shoes increased 15%, or more than $10 million, in September compared to August, which was itself a strong month of shoes on Amazon. Shoe sales continued to grow in September with athletic shoes accounting for more than $28 million in sales alone, significantly outperforming other categories, One Click Retail says. The Superfeet brand of insoles also posted healthy sales on Amazon, reaching more than $500,000 in revenue, thanks to ongoing Amazon promotions offering a 10%-15% discount on new and open-box insoles.

• No. 3: Toys

Sales of toys shot up by 15% month over month, increasing by about $10 million. Lego A/S led the charge, One Click Retail reports. Lego’s top three items alone—a model Porsche, a Millennium Falcon and Lego’s own Mindstorms EV3 robot—earned the company over $1 million in sales. Lego is No. 108 in the Internet Retailer 2016 Europe 500. The runner-up toy category on Amazon in September was infant/preschool toys. The Little Tykes-branded bouncer topped the list of best-selling infant/preschool toys.

• No. 4: Health and Personal Care

A broad category which spans from fitness to bath tissue, health and personal care item sales grew 5% in September compared to August, accounting for $5 million in incremental sales. FitBit branded fitness trackers accounted for three of the five top-selling products in the category.

• No. 5: Pet Care

Pet care has been growing steadily month-over-month and September was no different, One Click Retail reports. Pet care sales on Amazon grew about 5% month over month or by about $5 million. “As we’ve seen with paper towels, consumers are increasingly driven to e-commerce to purchase large or heavy items which are awkward or unpleasant to carry around the store,” the vendor writes in its analysis. As a result, dog food and cat litter performed well on Amazon in September, accounting for seven of the top 10 products in this category.

“Growth fluctuates greatly month over month and can be heavily skewed by promos and holiday shopping, so it’s important to take external factors into account when measuring the value of product groups,” One Click Retail writes in its blog post. “That being said, it’s times like these, the dead zones between formal shopping seasons, that reveal most accurately what consumers really buy.”

For example, in September, sales of software grew 16% compared with August. Personal care appliances grew 15% and musical instruments sales also grew 15%.  The burgeoning Amazon grocery space, meanwhile, is still dominated by beverages and coffee, making up 41% of the month’s total category sales, the vendor reports.

“What consumers buy online continues to evolve as people become more comfortable with and trusting of e-commerce,” One Click Retail says. “Relying too heavily on figures from back-to-school and from deal days such as Prime Day and Black Friday can skew the data and obscure what it is that’s driving most consumers, most of the time, to do their shopping online.”

One Click helps brands, including Panasonic Corp., Procter & Gamble Co. and Unilever, track sales of products at online retailers including Amazon, Walmart-Stores Inc. (No. 4 in the Top 500), Target Corp. (No. 22), Staples Inc. (No. 5) and The Home Depot Inc. (No. 7).